If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned. (John 15:6)
This verse is just one of a plethora of texts from the Bible refuting the various theologies of eternal security. Christ is not speaking of superficial believers, who like those discussed in 1 John 2:19 were not true believers, but instead, those who were truly regenerated believers who lost their salvation. When one examines the various instances of the term ἐν ἐμοί ("in me") in the Gospel of John that shows that those whom this term is applied to are salvifically united with Christ or Christ being "in" the Father:
He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him (ἐν ἐμοὶ μένει κἀγὼ ἐν αὐτω). (John 6:56)
But if I do, though ye believe not me, believe the works: they ye may know, and believe, that the Father is in me (εν εμοι), and I in him. (John 10:38)
Believest though not that I am in the Father, and the Father in me (εν εμοι)? The words that I speak unto you I speak not of myself: but the Father that dwelleth in me (εν εμοι), he doest the works. Believe me that I am in the Father, and the Father in me (εν εμοι): or else believe me for the very works' sake. (John 14:10-11)
At that day ye shall know that I am in the Father, and ye in me (εν εμοι), and I in you. (John 14:20)
Hereafter I will not talk much with you: for the prince of this world cometh, and has nothing in me (εν εμοι). (John 14:30)
Every branch in me (εν εμοι) that beareth not fruit he taketh away: and every branch that beareth fruit, he purgeth it, that it may bring forth more fruit . . . Abide in me (εν εμοι), and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me. I am in the vine, ye are the branches: He that abideth in me (εν εμοι), and I in him, the same bringeth forth much fruit: for without me ye can do nothing. If a man abide not in me (εν εμοι), he is cast forth as a branch and is withered; and men gather them, and cast them into the fire, and they are burned. if ye abide in me (εν εμοι), and my words abide in you, ye shall ask what ye will, and it shall be done unto you. (John 15:2, 5-7)
These things I have spoken unto you, that in me (εν εμοι) ye might have peace. In the world ye shall have tribulation: but be of good cheer; I have overcome the world. (John 16:33)
That they all may be one; as though, Father, art in me (εν εμοι), and I in thee; that they also may be one in us: that the world may believe that thou hast sent me . . . I in them, and thou in me (εν εμοι), that they may be made perfect in one; and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me. (John 17:21, 23)
As with many other key texts (e.g., Heb 6:4-6), the plain, exegetically-sound interpretation of John 15:6 is that true, regenerate believers can fall from salvation; in other words, the people in view are not a pseudo-branch or branch that was never truly connected to the vine; instead, Christ assumes that these people who fall away were already "in" Him and were a genuine branch of the vine, resulting in their being thrown in the fire (i.e., being lost). This is yet another refutation of many (false) theologies within Evangelical Protestantism. When one examines the biblical texts exegetically, the other theology that can be truly labelled “Biblical Christianity” is Latter-day Saint theology.