The DidacheTeaches: Father, Son, and Spirit are named together
c. 100 AD · Didache 7
Now about baptism, this is how you are to baptize: after saying all these things beforehand, baptize in the name of the Father and of the Son and of the Holy Spirit in living water. If you do not have living water, baptize in other water; and if you cannot do it in cold water, use warm.
Justin MartyrTeaches: Father, Son, and Spirit are named together
c. 100–c. 165 AD · First Apology 6
From this we are also called atheists. We confess that we are atheists with regard to the gods that people consider real, but not with regard to the truest God, the Father of justice, self-control, and the other virtues, a God completely unmingled with evil.
AthenagorasTeaches: Father, Son, and Spirit are named together
fl. c. 177 AD · Plea for the Christians 10
I have sufficiently demonstrated that we are not atheists, for we worship one God who is unbegotten, eternal, invisible, impassible, incomprehensible, and uncircumscribed. He is apprehended only by mind and reason, and encompassed by light, beauty, spirit, and indescribable power.
NovatianTeaches: Father, Son, and Spirit are named together
fl. c. 250 AD · On the Trinity 29
The logic of reason and the authority of faith, built on the Lord's own words and writings, urge us to believe in the Holy Spirit as well. The church was promised this Spirit long ago, but he was given at the appointed times. He was promised through the prophet Joel (Joel 2:28-29), but given through Christ.
NovatianRejects: Calling the Son God makes two Gods
fl. c. 250 AD · On the Trinity 30
We have spoken briefly about the Father, the Son, and the Holy Spirit, keeping our statement concise and avoiding lengthy debate, though we could have expanded far more and argued at greater length because the entire Old and New Testament bears witness that the true faith stands exactly as we hold it.
The same rule of truth teaches us to believe, after the Father, in the Son of God as well — Christ Jesus, our Lord and God, yet still the Son of God. This God is both one and alone, the Creator of all things, as I have already stated above.