You know how much EVERYONE makes such a deal about the Trinity, whether WE BELIEVE or those who spend decades denying it or those who spend 100's of years decrying it! I'm wondering if the early assembly didn't make such a deal of it, because they "got it." For us, it's almost like a secret code; we scour the Scriptures looking for the word that doesn't exist. We fight so hard to prove it and get into long arguments at our doorsteps with our brothers from the 19th Century.
I think it wasn't a big deal to the 1st Century saints simply because they all knew it! Listen to how Paul blithely posits this thought;
"Now He who establishes us with you in Christ and anointed us is God, Who also sealed us and gave the Spirit in our hearts as a pledge." (2 Cor 1:21-22)
No big deal, no fanfare...if you hunt up "trinity" verses, you come up with 26...or 20...or 7...or? Key for us; the key to the "Mystery" of the Gospel is the Trinity...we get it, and when we live it, it explains itself!
A Biblical manifest? One of my favorites is 2 Cor 4:7;
"But we have this treasure in earthen vessels, that the surpassing greatness of the power may be of God and not from ourselves."
When we bust up the Greek, this "dirt box" (From Genesis 2:7) is actually something that holds something, a chest if you will (Gr, ostrakinos-frail dirt. Gr, skeuos-vessel). What does it hold? A "treasure chest" (Gr, thesauros-a place where valuables are kept)
So, we have a chest or a treasure chest holding a treasure chest...hmmm, what's in the held treasure chest? Duh, Treasure! And what is the treasure?
"(We are) always carrying about in (our) body the dying of Jesus, that the Life of Jesus also may be manifested in our body." (2 Cor 4:10)
To get further carried away, we have God who built our body, we have Holy Spirit who holds the "chest" in our body and Jesus is in the held chest!
BING, BATTA, BOOM!
TRINITY!
IN US!