It's evident from their writings and by virtue of the fact that the trinity was not a thing until centuries after Christ died.
I keep saying the same thing and nobody disagrees that the trinity is an evolved doctrine that took centuries to finalize. Do you believe that?
Are you a Mormon?
What you can't seem to understand is that the Holy Trinity WAS believed by the early Christians even though the term "Trinity" may not have been strictly used and not formalized in a creed form until the Nicean Council. Here are but a few passages from some of the pre-Nicean fathers:
We will prove that we worship him reasonably; for we have learned that he is the Son of the true God himself, that he holds a second place, and the Spirit of prophecy a third. For this they accuse us of madness, saying that we attribute to a crucified man a place second to the unchangeable and eternal God, the Creator of all things; but they are ignorant of the mystery which lies therein. (Justin MartyrFirst Apology 13:56)
For this cause, yea and for all things, I praise Thee, I bless Thee, I glorify Thee, through the eternal and heavenly High-priest, Jesus Christ, Thy beloved Son, through whom with Him and the Holy Spirit be glory both now [and ever] and for the ages to come. Amen. Martyrdom of Polycarp 14:3
And at the same time the mystery of the oikonomia is safeguarded, for the unity is distributed in a Trinity. Placed in order, the three are the Father, Son, and Spirit. They are three, however, not in condition, but in degree; not in being, but in form; not in power, but in kind; of one being, however, and one condition and one power, because he is one God of whom degrees and forms and kinds are taken into account in the name of the Father, and of the Son, and of the Holy Spirit. TertullianAgainst Praxeas 2