John Salza on his Website (which appears to be offline, as of writing, but accessible via archive.org) has a good listing from the patristic literature against the various forms of eternal security many of our Protestant friends accept as a doctrine; while I take exception with Salza on many issues, including his poor grasp of Koine Greek, the quotations from the early Church Fathers is pretty sound. It also ties very well into passages such as Heb 6 and 10, as well as the example of King David which clearly show that a truly justified believer can lost their salvation:
"And pray ye without ceasing in behalf of other men; for there is hope of the repentance, that they may attain to God. For 'cannot he that falls arise again, and he may attain to God.'" Ignatius of Antioch, To the Ephesians, 10 ( A.D. 110).
"Watch for your life's sake. Let not your lamps be quenched, nor your loins unloosed; but be ye ready, for ye know not the hour in which our Lord cometh. But often shall ye come together, seeking the things which are befitting to your souls: for the whole time of your faith will not profit you, if ye be not made perfect in the last time." Didache, 16 (A.D. 90).
"And as many of them, he added, as have repented, shall have their dwelling in the tower. And those of them who have been slower in repenting shall dwell within the walls. And as many as do not repent at all, but abide in their deeds, shall utterly perish...Yet they also, being naturally good, on hearing my commandments, purified themselves, and soon repented. Their dwelling, accordingly, was in the tower. But if any one relapse into strife, he will be east out of the tower, and will lose his life." Hermas, The Shephard, 3:8:7 (A.D. 155).
"[T]hat eternal fire has been prepared for him as he apostatized from God of his own free-will, and likewise for all who unrepentant continue in the apostasy, he now blasphemes, by means of such men, the Lord who brings judgment [upon him] as being already condemned, and imputes the guilt of his apostasy to his Maker, not to his own voluntary disposition." Justin Martyr, fragment in Irenaeus' Against Heresies, 5:26:1 (A.D. 156).
"Now, in the beginning the spirit was a constant companion of the soul, but the spirit forsook it because it was not willing to follow. Yet, retaining as it were a spark of its power, though unable by reason of the separation to discern the perfect, while seeking for God it fashioned to itself in its wandering many gods, following the sophistries of the demons. But the Spirit of God is not with all, but, taking up its abode with those who live justly, and intimately combining with the soul, by prophecies it announced hidden things to other souls." Tatian the Syrian, To the Greeks, 13 (A.D. 175).
"Christ shall not die again in behalf of those who now commit sin, for death shall no more have dominion over Him; but the Son shall come in the glory of the Father, requiring from His stewards and dispensers the money which He had entrusted to them, with usury; and from those to whom He had given most shall He demand most. We ought not, therefore, as that presbyter remarks, to be puffed up, nor be severe upon those of old time, but ought ourselves to fear, lest perchance, after [we have come to] the knowledge of Christ, if we do things displeasing to God, we obtain no further forgiveness of sins, but be shut out from His kingdom. And therefore it was that Paul said, 'For if [God] spared not the natural branches, [take heed] lest He also spare not thee, who, when thou wert a wild olive tree, wert grafted into the fatness of the olive tree, and wert made a partaker of its fatness.'" Irenaeus, Against Heresies, 4:27:2 (A.D. 180).
"But some think as if God were under a necessity of bestowing even on the unworthy, what He has engaged (to give); and they turn His liberality into slavery. But if it is of necessity that God grants us the symbol of death, then He does so unwilling. But who permits a gift to be permanently retained which he has granted unwillingly? For do not many afterward fall out of (grace)? Is not this gift taken away from many?" Tertullian, On Repentance, 6 (A.D. 204).
"Confession is the beginning of glory, not the full desert of the crown; nor does it perfect our praise, but it initiates our dignity; and since it is written, 'He that endureth to the end, the same shall be saved,' whatever has been before the end is a step by which we ascend to the summit of salvation, not a terminus wherein the full result of the ascent is already gained." Cyprian, Unity of the Church, 21 (A.D. 251).
"Therefore, my beloved, we also have received of the Spirit of Christ, and Christ dwelleth in us, as it is written that the Spirit said this through the month of the Prophet: --I will dwell in them and will walk in them. Therefore let us prepare our temples for the Spirit of Christ, and let us not grieve it that it may not depart from us. Remember the warning that the Apostle gives us:--Grieve not the Holy Spirit whereby ye have been sealed unto the day of redemption. For from baptism do we receive the Spirit of Christ ... And whatever man there is that receives the Spirit from the water (of baptism) and grieves it, it departs from him until he dies, and returns according to its nature to Christ, and accuses that man of having grieved it." Aphrahat, Demonstrations, 6:14 (A.D. 345).
"Thou art made partaker of the Holy Vine. Well then, if thou abide in the Vine, thou growest as a fruitful branch; but if thou abide not, thou wilt be consumed by the fire. Let us therefore bear fruit worthily. God forbid that in us should be done what befell that barren fig-tree, that Jesus come not even now and curse us for our barrenness." Cyril of Jerusalem, Catechetical Lectures, I:4 (A.D. 350).
"It is the Spirit then which is in God, and not we viewed in our own selves; and as we are sons and gods because of the Word in us, so we shall be in the Son and in the Father, and we shall be accounted to have become one in Son and in Father, because that that Spirit is in us, which is in the Word which is in the Father. When then a man falls from the Spirit for any wickedness, if he repent upon his fall, the grace remains irrevocably to such as are willing; otherwise he who has fallen is no longer in God (because that Holy Spirit and Paraclete which is in God has deserted him), but the sinner shall be in him to whom he has subjected himself, as took place in Saul's instance; for the Spirit of God departed from him and an evil spirit was afflicting him." Athanasius, Discourse Against the Arians, 3:25 (A.D. 362).
"Clerics who are guilty of the sin unto death are degraded from their order, but not excluded from the communion of the laity." Basil, To Amphilochius, Letter 199:32 (A.D. 375).
"This temple is holier than that; for it glistened not with gold and silver, but with the grace of the Spirit, and in place of the ark and the cherubim, it had Christ, and His Father, and the Paraclete seated within. But now all is changed, and the temple is desolate, and bare of its former beauty and comeliness, unadorned with its divine and unspeakable adornments, destitute of all security and protection; it has neither door nor bolt, and is laid open to all manner of soul-destroying and shameful thoughts; and if the thought of arrogance or fornication, or avarice, or any more accursed than these, wish to enter in there is no one to hinder them; whereas formerly, even as the Heaven is inaccessible to all these, so also was the purity of thy soul." John Chrysostom, To the Fallen Theodore, Letter 1 (A.D. 378).
"But these sins were not after Baptism, you will say. Where is your proof? Either prove it--or refrain from condemning; and if there be any doubt, let charity prevail. But Novatus, you say, would not receive those who lapsed in the persecution. What do you mean by this? If they were unrepentant he was right; I too would refuse to receive those who either would not stoop at all or not sufficiently, and who would refuse to make their amendment counterbalance their sin; and when I do receive them, I will assign them their proper place; but if he refused those who wore themselves away with weeping, I will not imitate him." Gregory of Nazianzen, Oration on the Holy Lights, 39:19 (A.D. 381).
"Let us admonish each other. Let us correct each other, that we may not go to the other world as debtors, and then, needing to borrow of others, suffer the fate of the foolish virgins, and fall from immortal salvation." John Chrysostom, Concerning Statues, 21 (A.D. 387).
"Some offences are light, some heavy. It is one thing to owe ten thousand talents, another to owe a farthing. We shall have to give account of the idle word no less than of adultery; but it is not the same thing to be put to the blush, and to be put upon the rack, to grow red in the face and to ensure lasting torment. Do you think I am merely expressing my own views? Hear what the Apostle John says: 'He who knows that his brother sinneth a sin not unto death, let him ask, and he shall give him life, even to him that sinneth not unto death. But he that hath sinned unto death, who shall pray for him? 'You observe that if we entreat for smaller offences, we obtain pardon: if for greater ones, it is difficult to obtain our request: and that there is a great difference between sins.'" Jerome, Against Jovianus, 2:30 (A.D. 393).
"And, consequently, both those who have not heard the gospel, and those who, having heard it and been changed by it for the better, have not received perseverance, and those who, having heard the gospel, have refused to come to Christ, that is, to believe on Him, since He Himself says, 'No man cometh unto me, except it were given him of my Father,' and those who by their tender age were unable to believe, but might be absolved from original sin by the sole layer of regeneration, and yet have not received this laver, and have perished in death: are not made to differ from that lump which it is plain is condemned, as all go from one into condemnation." Augustine, On Rebuke and Grace, 12 (A.D. 427).
"The faith of these, which worketh by love, either actually does not fail at all, or, if there are any whose faith fails, it is restored before their life is ended, and the iniquity which had intervened is done away, and perseverance even to the end is allotted to them. But they who are not to persevere, and who shall so fall away from Christian faith and conduct that the end of this life shall find them in that case, beyond all doubt are not to be reckoned in the number of these, even in that season wherein they are living well and piously. For they are not made to differ from that mass of perdition by the foreknowledge and predestination of God, and therefore are not called according to God's purpose, and thus are not elected.” Augustine, On Rebuke and Grace, 16 (A.D. 427).
"It is, indeed, to be wondered at, and greatly to be wondered at, that to some of His own children--whom He has regenerated in Christ--to whom He has given faith, hope, and love, God does not give perseverance also." Augustine, On Rebuke and Grace, 18 (A.D. 427).
"Let the inquirer still go on, and say, 'Why is it that to some who have in good faith worshipped Him He has not given to persevere to the end?' Why except because he does not speak falsely who says, 'They went out from us, but they were not of us; for if they had been of us, doubtless they would have continued with us.' Are there, then, two natures of men? By no means. If there were two natures there would not be any grace, for there would be given a gratuitous deliverance to none if it were paid as a debt to nature. But it seems to men that all who appear good believers ought to receive perseverance to the end. But God has judged it to be better to mingle some who would not persevere with a certain number of His saints, so that those for whom security from temptation in this life is not desirable may not be secure." Augustine, On the Gift of Perseverance, 19 (A.D. 429).
"The manifold mercy of God so assists men when they fall, that not only by the grace of baptism but also by the remedy of penitence is the hope of eternal life revived, in order that they who have violated the gifts of the second birth, condemning themselves by their own judgment, may attain to remission of their crimes, the provisions of the Divine Goodness having so ordained that God’s indulgence cannot be obtained without the supplications of priests. For the Mediator between God and men, the Man Christ Jesus, has transmitted this power to those that are set over the Church that they should both grant a course of penitence to those who confess, and, when they are cleansed by wholesome correction admit them through the door of reconciliation to communion in the sacraments." Pope Leo the Great [regn. A.D. 440-461], To Theodore, Epistle 108:2 (A.D. 452).
"The branches of the vine. Thus there are branches in the vine, not that they may bestow anything upon the vine, but that they may receive from it the means by which they may live...And by this it is an advantage to the disciples, not to Christ, that each have Christ abiding in him, and that each abide in Christ. For if the branch is cut off, another can sprout forth from the living root; but that which has been cut off, cannot live without the root." Council of Orange, Canon 24 (A.D. 529).
"And they who mourn their transgressions certainly cast forth by confession the wickedness with which they have been evilly satiated, and which oppressed the inmost parts of their soul; and yet, in recurring to it after confession, they take it in again. But the sow, by wallowing in the mire when washed, is made more filthy. And one who mourns past transgressions, yet forsakes them not, subjects himself to the penalty of more grievous sin, since he both despises the very pardon which he might have won by his weeping, and as it were rolls himself in miry water; because in withholding purity of life from his weeping he makes even his very tears filthy before the eyes of God." Pope Gregory the Great [regn. A.D. 590-604], Pastoral Rule, 30 (A.D. 591).
"The remission of sins, therefore, is granted alike to all through baptism: but the grace of the Spirit is proportional to the faith and previous purification. Now, indeed, we receive the first fruits of the Holy Spirit through baptism, and the second birth is for us the beginning and seal and security and illumination s of another life. It behooves as, then, with all our strength to steadfastly keep ourselves pure from filthy works, that we may not, like the dog returning to his vomit, make ourselves again the slaves of sin. For faith apart from works is dead, and so likewise are works apart from faith. For the true faith is attested by works." John of Damascus, On the Orthodox Faith, 4:9 (A.D. 743).