Leaving aside for a moment the argument of whether of not there is a trinity, I would like to address a central aspect of the question. One of the most damaging aspects about the trinity debate, one to which many sincere Trinitarian Christians adhere, is that if someone does not believe in the trinity they are not saved.
Salvation is the most important thing in life. Indeed, it determines one's eternal state. Why would God not make a requirement to believe in the trinity as plain as the nose on one's face? Why would God make it necessary to "read between the lines" in order be eternally saved? I can't imagine an earthly father doing something like that, let alone our heavenly father. If it were that important why would God not have used the word "trinity" at least once somewhere in the scriptures? Why do we not find even once the phrase, "Jesus is God" somewhere in the scriptures? Why would He make it so darn cryptic?
To those who insist belief in the trinity is a requirement for salvation, I challenge anyone to find an actual scripture that explicitly says one must believe in the trinity to be saved. In fact I will allow the introduction of any extraneous document written before the 5th century to prove the requirement of belief in the trinity to be saved, even though I personally take 100% of my doctrine from the scriptures and nothing but the scriptures. Now I understand that there are extraneous church documents that expound on the trinity, but I've not seen any that actually make belief in the trinity a requirement for salvation. At least the early trinitarians were tolerant of those who did not believe in the trinity, not condemning them to hell, as is the common teaching in the orthodox churches these days.
So let's see some scriptures that make belief in the trinity a requirement for salvation.
To answer your question,
You said ( Why would God not make a requirement to believe in the trinity as plain as the nose on one's face?)
All because God gave each individual a brain to think with, God is not going to do all your work for you.That means studying the Bible for ones self. Don't Waite to be spoon fed.
There are lot people as well as Christians that don't even have the slightest clue or idea what the bible actually does teaches or Confirm's.
There are things in the bible that are as plain as the nose on ones face.
In your speaking about the Trinity. Just how much more can Christ Jesus be about it, Than when he was speaking to the disciples, saying unto them "Go you therefore, teach all nation's, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit" Matthew 28:19.
"For in him ( Jesus) dwells all the fullness of the Godhead bodily" Colossians 2:9.
There you have the Trinity, Father,Son,Holy Spirit.
How much more can that be plain as the nose on ones face.
So the requirement for salvation is to be baptized in the Father and Son and the Holy Spirit. For salvation.
Jesus in the speaking to the disciples saying unto them "If you had known me, you should have known my Father also,
and from here forth, you know him, and have seen him" John 14:7.
Note Jesus said ( and have seen him)
All because Jesus is God the Father.
How much more plainly does Jesus have to be, that it's as plain as the nose on ones face.
Here's a question for anyone, In the book of John 14:28---"Ye have heard how I said unto you, I go away, and come again unto you. If ye loved me, ye would rejoice, because I said, I go unto the Father: for my Father is greater than I "
How is the Father greater than Jesus,
If Jesus is God the Father, how exactly does this work?
It's as plain as the nose on ones face.