Irenaeus was a bishop and theologian during the 2nd century. His 5 part work entitled Against Heresies gives us a view into the early church. As Christmas approaches, here are some of his thoughts on the Incarnation (III.20).

Just as the physician is proved by his patients, so is God also revealed through men. And therefore Paul declares, “For God has concluded all in unbelief, that He may have mercy upon all” (Romans 11:32) [saying this in reference to man], who had been disobedient to God, and being cast off from immortality, then obtained mercy, receiving through the Son of God that adoption which is [accomplished] by Himself.

This adoption, which is a work of God, is granted to all who have an active faith in God and the salvation that comes through Christ.

For he who holds, without pride and boasting, the true glory (opinion) regarding created things and the Creator, who is the Almighty God of all, and who has granted existence to all; [such an one,] continuing in His love and subjection (obedience), and giving of thanks, shall also receive from Him the greater glory of promotion, looking forward to the time when he shall become like Him who died for him,

But this promotion is only possible because the Word was willing to become flesh (John 1:1,14). And to suffer and die that we might be healed (Isaiah 53:5). For “it was expedient for one man to die on behalf of the people” (John 11:50; 18:14).

On this account, therefore, the Lord Himself, who is Emmanuel from the Virgin, (Isaiah 7:4) is the sign of our salvation, since it was the Lord Himself who saved them, because they could not be saved by their own instrumentality; and, therefore, when Paul sets forth human infirmity, he says: “For I know that there dwells in my flesh no good thing,” (Romans 7:18) showing that the “good thing” of our salvation is not from us, but from God. And again: “Wretched man that I am, who shall deliver me from the body of this death?” (Romans 7:24)

Then he introduces the Deliverer, [saying,] “The grace of Jesus Christ our Lord.” And Isaiah declares this also, [when he says:] “Be strengthened, you hands that hang down, and you feeble knees; be encouraged, you feeble-minded; be comforted, fear not: behold, our God has given judgment with retribution, and shall recompense: He will come Himself, and will save us.” (Isaiah 25:3) Here we see, that not by ourselves, but by the help of God, we must be saved.

Again, that it should not be a mere man who should save us, nor [one] without flesh— for the angels are without flesh— [the same prophet] announced, saying: “Neither an elder, nor angel, but the Lord Himself will save them because He loves them, and will spare them: He will Himself set them free.” (Isaiah 63:9)

Disobedient and fallen man was in no position to restore the relationship with God nor escape a just condemnation. This salvation was only made possible when the Father sent His Son Jesus to earth which is “good news of great joy which will be for all the people” (Luke 2:10).

But this good news must be preached and the hearers must avail themselves of the opportunity to choose whether they will be gathered into the arms of God and adopted as sons of God or not.

Irenaeus explains (IV.37):

This expression [of our Lord], “How often would I have gathered your children together, and you would not,” (Matthew 23:37) set forth the ancient law of human liberty, because God made man a free [agent] from the beginning, possessing his own power, even as he does his own soul, to obey the behests of God voluntarily, and not by compulsion of God. For there is no coercion with God, but a good will [towards us] is present with Him continually. And therefore does He give good counsel to all. And in man, as well as in angels, He has placed the power of choice …

… in faith, has God preserved the will of man free and under his own control, saying, “According to your faith be it unto you;” (Matthew 9:29) … man is in his own power with respect to faith. And for this reason, “he that believes in Him has eternal life while he who believes not the Son has not eternal life, but the wrath of God shall remain upon him.” (John 3:36)