Matthew 5:48 as implicit evidence for the Latter-day Saint understanding of Theosis

Be ye therefore perfect, even as your Father which is in heaven is perfect. (Matt 5:48)

Some critics of Latter-day Saint soteriology argue that, as Jesus is speaking of moral perfection, such is inconsistent with LDS views of theosis. Firstly, it should be noted that moral perfection is a natural prerequisite of LDS deification, so even if such were the case, it implicitly supports Latter-day Saint theology. While it doesn't encapsulate LDS views of theosis, it certainly embraces such. Luke 6:36 stands as the parallel verse:

Be ye therefore merciful, as your Father also is merciful.

It seems more probable that in this instance, Christ was making a parallel with known Scripture, "Ye shall be holy: for I the Lord your God am holy" (Lev 19:2). "To be holy" (Greek: αγιος; Hebrew: קדושׁ) implies far more than "be merciful," and this corresponds well with the Latter-day Saint understanding of perfection or completeness--to say nothing of sanctification as theosis was commonly described in the writings of early Christians, including the following (references are to the 10-voume Ante-Nicene Fathers set by Philip Schaff; W = Western writer; E = Eastern writer):

We have learned that only those who have lived close to God in holiness and virtue are deified.  Justin Martyr (c. 160, C), 1.170.

Neither, then, immortal nor yet mortal did He make man. Rather, as we have said above, man was capable of both. If he would incline to the things of immortality, keeping the commandment of God, he would receive immortality as a reward from Him. And he would become divine. Theophilus (c. 180, E), 2.105.

And again, “God stood in the congregation of the gods; He judges among the gods.” He refers to the Father and the son, and to those who have received the adoption. Irenaeus (c. 180, E/W), 1.448.

As I have already said, He caused man to cleave to and to become one with God … unless man had been joined to God, he could never have become a partaker of incorruptibility. Irenaeus (c. 180, E/W), 1.448.

How will man pass into God, unless God had first passed into man? Irenaeus (c. 180, E/W), 1.507

Human are not uncreated. But by their being in existence throughout a long course of ages, they will receive a faculty of the Uncreated. This will be through the free bestowal of eternal existence upon them by God…. But being subjection to God is continuance in immortality, and immortality is the glory of the uncreated One. Irenaeus (c. 180, E/W), 1.521.

It must be that you should partake of the nature of man at the outset, and then afterwards partake of the glory of God. Irenaeus (c. 180, E/W), 1.523.

Our Lord Jesus Christ, through His transcendent love, became what we are, so that He might bring us to be even what He Himself is. Irenaeus (c. 180, E/W), 1.526.

The Lord thus has redeemed us through His own blood, giving His soul for our souls, and His flesh for our flesh. He has also poured out the Spirit of the Father for the union and communion of God and man. He indeed imparts other hand, He has attached man to God by His own incarnation. He bestowed true and enduring immortality upon us at his coming—by means of communion with God. Irenaeus (c. 180, E/W), 1.527.

Being baptized, we are illuminated. Illuminated, we become sons. Being made sons, we are made perfect. Being made perfect, we are made immortal. He says, “I have said that you are gods, and all are sons of the Highest.” Clement of Alexandria (c. 195, E), 2.215.

In this way, it is possible for man to God already to have become divine. “I said, you are gods, and sons of the highest” Clement of Alexandria (c. 195, E), 2.437.

The man of God is consequently divine and is already holy. He is God-bearing and God-borne. Clement of Alexandria (c. 195, E), 2.547.

It would be impossible that another God could be admitted, when it is permitted to no other being to possess anything of God. Well, then, you say, at that rate we ourselves possess nothing of God. But indeed we do, and will continue to do so. Only, it is from Him that we receive it, and not from ourselves. For we will be even gods, if we deserve to be among those of whom He declared, “I have said, ‘You are gods,’” and “God stands in the congregation of gods.” But this comes of His own grace, not from any property in us. For it is He alone who can make gods. Tertullian (c. 200, W), 3.480.

You will be a companion of God, and a co-heir with Christ, no longer enslaved by lusts or passions and wasted by disease. For you have become divine…. God has promised to bestow these upon you, for you have been deified and begotten unto immortality. Hippolytus (c. 225, W), 5.153.

I am of the opinion that the expression, “god will be all in all,” means that He will be “all” in each individual person. Now, He will be “all” in each individual in this way: when everything that one can either fell, understand, or think will be wholly God. This will be when a person has been cleansed from the dregs of every sort of vice, and has every cloud of wickedness completely swept away…. It is when God will be measure and standard of all movements. Thus, God will be “all,” for there will no longer be any distinction of good and evil—for evil will exist nowhere. Origen (c. 225, E), 4.345.

It is one and the same thing to have a share in the Holy Spirit, which is of the Father and the Son. For the nature of he Trinity is one and incorporeal. And what we have said regarding the participation of the soul is to be understood also of angels and heavenly creature needs a participation in the Trinity. Origen (c. 225, E), 4.379.

They see that from Him there began the union of the divine with the human nature. This was so that the human—by communion with the divine—might rise to be divine. This not only happened in Jesus, but also in all those who not only believe, but enter upon the life what Jesus taught. Origen (c. 248, E), 4.475.

There are honored by God—through His Only-Begotten Word—by participation in His divinity and thereby also His name. Origen (c. 248, E), 4.479.

What man is, Christ was willing to be—so that man also be what Christ is. Cyprian (c. 250, W), 5.468.

What Christ is, we Christians will be, if we imitate Christ. Cyprian (c. 250, W), 5.469; see also 1.463. 1.522, 1.533.

With respect to Matt 5:48 itself, "be ye" is ἔσεσθε, the indicative future middle deponent second plural person of the verb "to be" (ειμι).  In other words, it refers to something that we must do ourselves--it will not be done for us (let alone imputed to us based on an alien righteousness!) It is something one must be actively engaged in, though we are not expected to be at that point at this very moment. In short, one must gradually and consistently, seek to become perfect or fully developed, like God the Father (and, in his post-exalted state [cf. Phil 2:5-11], Jesus, as we learn in 3 Nephi 12:48]). It is only through the saving ordinances, as administered by and through The Church of Jesus Christ of Latter-day Saints, and by the grace of God, can this be accomplished (notwithstanding the pathetic arguments otherwise [see here and here for a response to one critic]).