Paul and John were both religious Jews, and therefore strongly monotheistic. So what did they mean by phrases like:
"For in him [the Son] all things were created ... he is the head of the body, the church ... the firstborn from the dead ... God was pleased to have all his fulness dwell in him,"from Paul's letter to the Colossian church; and
"In the beginning was the Word, and the Word was with God, and the Word was God ... The Word became flesh and made his dwelling among us ... the one and only Son, who came from the Father,"from the opening of John's Gospel?
It took several hundred years of argument, but in the end the church settled on the idea of a Trinitarian God - one God in three Persons - and that Jesus was both fully God and fully man, in one person.
Jews don't like this solution, Muslims don't like it either, and an assortment of Christian offshoots over the centuries have rejected it as well. So why is mainstream Christianity so attached to the idea of a Trinitarian God, what does it mean, and how is it monotheistic?
In my view, at the heart of the issue is the reality that God is essentially unknowable. How can anyone get their heads around a being who created the whole space-time universe out of nothing? If He created time, when did He do it; if He created space where was He when He did it? God is beyond space and time, but we have no way of understanding that. It's a reality which Kierkegaarde and Karl Barth described as infinite qualitative distinction (that Wikipedia reference isn't very good - if you know a better link please let me know).
So how can we talk about God as a person (or even as three persons)? How can a being beyond space and time relate to ordinary people in any meaningful sense, and how can we engage with, or even worship, such a being?
The answer has to be through God's self-revelation. We can't understand God (someone once said that anyone who claims to understand their God worships an idol). Nevertheless God can reveal himself to us - in a necessarily limited way - such that we can engage with that revelation. The Abrahamic religions understand that self-revelation to be through their scriptures and through creation, God's handiwork.
To Jews God reveals Himself through the Torah (and the Prophets), to Muslims through the Quran, and to Christians through the Bible. Scriptures are famously obscure and ambiguous in parts, so that revelation is modulated by the universe around us - God's creation - and by the way God has dealt over the centuries with His people.
Note, incidentally, the problems of language in describing God: God doesn't have gender, yet his self-revelation in the various scriptures is personal, so none of 'he', 'she' or 'it' really works well, but one must use something. So I'm using the traditional 'He', in spite of its limitations.
One self-revelation equates to one understanding of God which sits comfortably with monotheism. But what if God reveals himself in other, significantly different, ways?
What if God somehow revealed himself in a person? A person who in some mysterious way was both fully human and fully an expression of the one God? We can't begin to understand how that works of course, but then God is essentially not understandable - all we can do is appreciate his presentation(s) of Himself to us. A person is qualitatively different from a book; a human being localised in time and space has to be understood differently from the creator of that time and space. One God has revealed Himself in two fundamentally different ways.
Then there is the Christian experience of some sort of inner presence within them, which in some mysterious way unites them with other Christians, with Jesus, and with the otherwise unknowable God. A kind of Spirit which connects the human and the divine - inevitably almost indescribable and incomprehensible, but subjectively experienced as God revealing himself to us.
Three ways in which one God reveals himself to mankind - to you and I if we have eyes to see, ears to hear, heart and mind to experience. Creator and author, redeeming human being, and uniting restoring Spirit. One God revealing Himself through three 'persons' - self-expressions with whom limited humanity can relate.
A Trinitarian God who remains one God indivisible.