Ah, you have many questions.
Trinity or the concept of the Trinity was termed at the Council of Nicaea.
Early Church Fathers both Eastern, and Western Groups got together
to debate the rising tide of Heresy. Heresy occurs when someone begins
to spread false teaching on Christianity. They coined the term Trinity in
its simplest form 3 in 1, the nature of God.
Water is an analogy or metaphor to explain how the Trinity is possible.
Just to show you that it is possible. Let's go back to the Hebrew language.
The God called YHWH first appears to the Hebrew nation.
They called him Elohim. The term Elohim means 'One' God, but it is a little
more complicated. God is not purely singular.
He is HaShem (His Name)
He has a Ruach ( His Spirit)
He also appeared as Mosiach ( Isa Al-Masih). He actually appears several
times in the Tanakh (Old Testament). Burning Bush in Exodus, in Daniels
as the Angel of the Lord, etc,etc,etc
Read Isaiah 53 on the coming of Al-Masih. HaShem refers to him as
'HIS ARM'. Yurezowa Yahweh. Not messenger, not angel, etc,etc,etc
Original Sin occurred in the Garden of Eden. Adam and Eve disobeyed God's
command not to eat from the tree of knowledge. Therefore all generations
are infected by this disease called Sin per the prophet Isaiah.
Why is there a need for the Cross ? because Sin has to be atoned. The Hebrews
did that in the past.
Indeed, under the law almost everything is purified with blood, and without the
shedding of blood there is no forgiveness of sins. ~ hebrews 9:22
As Christians, we are all priests. Church roles are man made religious institutions.
Nothing evil about that. It is just a way to organize a Church body.
In the Name Yeshua Al-Masih