Since Jesus had two natures, one divine and one human, is human nature included in the Trinity, or just the divine nature? What did the early Christians and the traditions have to say about this?
"Is human nature included in the Trinity?" Included it what sense? Everything that is part of God is God, since God is supremely simple. But God is certainly not human nature, although He contains all the perfections of human nature (and more). The Second Person of the Holy Trinity assumed (took up) a human nature.
As St. Thomas says in "Whether the union of the Incarnate Word took place in the Person?":
The Word of God "did not assume human nature in general, but 'in atomo'"—that is, in an individual—as Damascene says (De Fide Orth. iii, 11); otherwise, every man would be the Word of God even as Christ was.
In the question "Whether the human nature was united to the Word of God accidentally?" of St. Thomas Aquinas's Summa Theologica, he states:
the Catholic faith, holding the mean between the aforesaid positions,* does not affirm that the union of God and man took place in the essence or nature, nor yet in something accidental [like clothing is to a man], but midway, in a subsistence or hypostasis.
*[heresies on the Incarnation like Nestorianism]
Also, one could argue this way:
- God is not a body.
- A human is a body and soul.
- Therefore, God is not a human.
Per John 1:1, "In the beginning was the Word, and the Word was with God, and the Word was God." The Word of course was Jesus. Also, from Genesis we see mainly God and the Holy Spirit, the other co-existence of the triune God: "...and the Spirit of God was moving over the surface of the waters.…" So Jesus is one with God, although he co-existed as a human being only while He was on this earth. So no, human nature is not included in the Trinity at all, in my belief....
One of the primary sources of the people at that time and which is still existing outside of the Bible by such as St. Paul in his many epistles, is Josephus, who wrote in 93 A.D./C.E. the following [I have the volume at home, but Googled it just now, for which I express my regrets in advance]:
"About this time there lived Jesus, a wise man, if indeed one ought to call him a man. For he was one who performed surprising deeds and was a teacher of such people as accept the truth gladly. He won over many Jews and many of the Greeks. He was the Messiah. And when, upon the accusation of the principal men among us, Pilate had condemned him to a cross, those who had first come to love him did not cease. He appeared to them spending a third day restored to life, for the prophets of God had foretold these things and a thousand other marvels about him. And the tribe of the Christians, so called after him, has still to this day not disappeared." - Jewish Antiquities, 18.3.3 §63 (Based on the translation of Louis H. Feldman, The Loeb Classical Library.) http://www.josephus.org/testimonium.htm