Here’s a great post from Tough Questions Answered. They find evidence for the Incarnation and the Trinity in the writings of Ignatius, who was the third Bishop of Antioch from 70 AD to 107 AD.
Here’s the raw quote from Ignatius’ “Epistle to the Ephesians”:
But our Physician is the only true God, the Father and Begetter of the only-begotten Son. We have also as a Physician the Lord our God, Jesus the Christ, the only-begotten Son and Word, before time began, but who afterwards became also man, of Mary the virgin. For “the Word was made flesh.” Being incorporeal, He was in a body; being impassible, He was in a passible body; being immortal, He was in a mortal body; being life, He became subject to corruption, that He might free our souls from death and corruption, and heal them, and might restore them to health, when they were diseased with ungodliness and wicked lusts.
And TQA discusses the passage:
There are several aspects of this passage which demonstrate that Saint Ignatius held beliefs consistent with the Doctrines of the Trinity and the Dual Nature of Christ. First, he refers to two separate Persons, God the Father and Jesus Christ, yet he calls both of them God.
[…]Second, Ignatius refers to Jesus Christ as begotten “before time began”. This is almost word for word identical to the Nicene Creed, which says, “I believe in. . . one Lord, Jesus Christ, the Son of God, the only-begotten, begotten of the Father before all ages. . .” Some today claim that the Early Church believed Christ’s being ”begotten” of the Father was in relation to His birth from Mary (specifically, this is an LDS claim). However, Ignatius’ comment here demonstrates that the Early Church’s understanding of Christ’s nature as “only-begotten” was a relationship with the Father that was “before time began” and has nothing to do with His earthly incarnation. It is interesting to note that the Greek word translated as “only-begotten” both here and in the New Testament is ”monogenes”. Monogenes literally means “one of a kind,” and to the Church Fathers it connoted Christ being of the same nature as the Father. . . something that was entirely unique to Him.
In addition to calling Christ God and claiming Him to be the “only-begotten” of the Father “before time began”, Ignatius tells us that “afterwards” Christ “became man”. Ignatius then goes on to point out some aspects that Christ’s becoming man added to His nature. He says that although Christ was incorporeal, He was in a body; although He was impassible, He was in a passible body; although He was immortal, He was in a mortal body; although He was life, He became subject to corruption. These differing aspects of Christ’s nature, aspects that are polar opposites to one another, speak to Christ having two natures, one as God and one as man, and demonstrate that Saint Ignatius understood Christ in this manner. As God, Christ was incorporeal, impassible, immortal, and life itself. However, as man He was corporeal, passible, mortal, and subject to corruption.