1 For in no other way could we have learned the things of God, unless our Master, existing as the Word, had become man. For no other being had the power of revealing to us the things of the Father, except His own proper Word. For what other person “knew the mind of the Lord,” or who else “has become His counsellor?” Again, we could have learned in no other way than by seeing our Teacher, and hearing His voice with our own ears, that, having become imitators of His works as well as doers of His words, we may have communion with Him, receiving increase from the perfect One, and from Him who is prior to all creation. We — who were but lately created by the only best and good Being, by Him also who has the gift of immortality, having been formed after His likeness (predestinated, according to the prescience of the Father, that we, who had as yet no existence, might come into being), and made the first-fruits of creation — have received, in the times known beforehand, [the blessings of salvation] according to the ministration of the Word, who is perfect in all things, as the mighty Word, and very man, who, redeeming us by His own blood in a manner consonant to reason, gave Himself as a redemption for those who had been led into captivity. And since the apostasy tyrannized over us unjustly, and, though we were by nature the property of the omnipotent God, alienated us contrary to nature, rendering us its own disciples, the Word of God, powerful in all things, and not defective with regard to His own justice, did righteously turn against that apostasy, and redeem from it His own property, not by violent means, as the [apostasy] had obtained dominion over us at the beginning, when it insatiably snatched away what was not its own, but by means of persuasion, as became a God of counsel, who does not use violent means to obtain what He desires; so that neither should justice be infringed upon, nor the ancient handiwork of God go to destruction. Since the Lord thus has redeemed us through His own blood, giving His soul for our souls, and His flesh for our flesh, and has also poured out the Spirit of the Father for the union and communion of God and man, imparting indeed God to men by means of the Spirit, and, on the other hand, attaching man to God by His own incarnation, and bestowing upon us at His coming immortality durably and truly, by means of communion with God—all the doctrines of the heretics fall to ruin.
2. Vain indeed are those who allege that He appeared in mere seeming. For these things were not done in appearance only, but in actual reality. But if He did appear as a man, when He was not a man, neither could the Holy Spirit have rested upon Him—an occurrence which did actually take place— as the Spirit is invisible; nor, [in that case], was there any degree of truth in Him, for He was not that which He seemed to be. But I have already remarked that Abraham and the other prophets beheld Him after a prophetical manner, foretelling in vision what should come to pass. If, then, such a being has now appeared in outward semblance different from what he was in reality, there has been a certain prophetical vision made to men; and another advent of His must be looked forward to, in which He shall be such as He has now been seen in a prophetic manner. And I have proved already, that it is the same thing to say that He appeared merely to outward seeming, and [to affirm] that He received nothing from Mary. For He would not have been one truly possessing flesh and blood, by which He redeemed us, unless He had summed up in Himself the ancient formation of Adam. Vain therefore are the disciples of Valentinus who put forth this opinion, in order that they my exclude the flesh from salvation, and cast aside what God has fashioned.
3. Vain also are the Ebionites, who do not receive by faith into their soul the union of God and man, but who remain in the old leaven of [the natural] birth, and who do not choose to understand that the Holy Ghost came upon Mary, and the power of the Most High did overshadow her: wherefore also what was generated is a holy thing, and the Son of the Most High God the Father of all, who effected the incarnation of this being, and showed forth a new [kind of] generation; that as by the former generation we inherited death, so by this new generation we might inherit life. Therefore do these men reject the commixture of the heavenly wine, and wish it to be water of the world only, not receiving God so as to have union with Him, but they remain in that Adam who had been conquered and was expelled from Paradise: not considering that as, at the beginning of our formation in Adam, that breath of life which proceeded from God, having been united to what had been fashioned, animated the man, and manifested him as a being endowed with reason; so also, in [the times of] the end, the Word of the Father and the Spirit of God, having become united with the ancient substance of Adam's formation, rendered man living and perfect, receptive of the perfect Father, in order that as in the natural [Adam] we all were dead, so in the spiritual we may all be made alive. For never at any time did Adam escape the hands of God, to whom the Father speaking, said, “Let Us make man in Our image, after Our likeness.” And for this reason in the last times (fine), not by the will of the flesh, nor by the will of man, but by the good pleasure of the Father, His hands formed a living man, in order that Adam might be created [again] after the image and likeness of God.
Source: Adversus haereses (New Advent)