On The Holy Trinity by St. Augustine
Chapter 18.--Whether the Sentence of John is to Be Understood of Our Future Likeness with the Son of God in the Immortality Itself Also of the Body.
24. But the Apostle John says, |Beloved, now are we the sons of God; and it doth not yet appear what we shall be: but we know that, when He shall appear, we shall be like Him, for we shall see Him as He is.| Hence it appears, that the full likeness of God is to take place in that image of God at that time when it shall receive the full sight of God. And yet this may also possibly seem to be said by the Apostle John of the immortality of the body. For we shall be like to God in this too, but only to the Son, because He only in the Trinity took a body, in which He died and rose again, and which He carried with Him to heaven above. For this, too, is called an image of the Son of God, in which we shall have, as He has, an immortal body, being conformed in this respect not to the image of the Father or of the Holy Spirit, but only of the Son, because of Him alone is it read and received by a sound faith, that |the Word was made flesh.| And for this reason the apostle says, |Whom He did foreknow, He also did predestinate to be conformed to the image of His Son, that He might be the first-born among many brethren.| |The first-born| certainly |from the dead,| according to the same apostle; by which death His flesh was sown in dishonor, and rose again in glory. According to this image of the Son, to which we are conformed in the body by immortality, we also do that of which the same apostle speaks, |As we have borne the image of the earthy, so shall we also bear the image of the heavenly;| to wit, that we who are mortal after Adam, may hold by a true faith, and a sure and certain hope, that we shall be immortal after Christ. For so can we now bear the same image, not yet in sight, but in faith; not yet in fact, but in hope. For the apostle, when he said this, was speaking of the resurrection of the body.