Why believe in something that isn’t in the Bible??

This is a conversation between two people.

A: The Bible only lists God’s three works. Inventing a conditional formula like 1 = 3, 3 = 1,
     or claiming that each is 100% God, turns it into heresy like Protestantism or Catholicism.

B: Your claim is the heresy of Modalism. Stop spreading false, demonic teachings.
     The Athanasian Creed already explains this well.

A: Whether it’s Modalism or Tritheism doesn’t matter.
 Why believe in something that isn’t in the Bible??

People say “it’s not in the Bible,” yet they insist on and believe things as if they were in the Bible.
Or regardless of what the Bible actually says, they simply believe whatever someone tells them.
Or they don’t even really care what kind of god they believe in.

Yet some people say, “Why believe in what isn’t in the Bible?”
If something is not in the Bible, it means the concept itself does not exist in the Bible.
To believe it unconditionally anyway is blind faith and idolatry.

God does not reveal Himself in a difficult or confusing way. He is this clear and precise.

I, even I, am the LORD, and apart from Me there is no savior. (Isaiah 43:11)

I am the first and I am the last; besides Me there is no god. (Isaiah 44:6)

I made the earth and created man on it; it was My hands that stretched out the heavens,
and I commanded all their host. (Isaiah 45:12)

Yet for us there is but one God, the Father, from whom all things came and for whom we live;
and there is but one Lord, Jesus Christ, through whom all things came and through whom we live. (1 Corinthians 8:6)