All you need to believe is that what happened in the “Old Testament” was fundamentally changed by the New One, that God isn’t “One” anymore but rather “three, or two, and a son, also a god, and some other holy holy holy things,” and that nothing at all happened on Mount Sinai (or that God changed his mind,) and that certain graven images are perfectly acceptable now that there’s a new covenant whose precepts are based on writings in... The new covenant. Then you get to go to a new, private heaven, because you, in accordance with your own book, your own history, and your own beliefs, have the exclusive key to the kingdom. And that is why, my brothers and sisters, YOU will ascend at your rapture!
We aren’t in the end times - the end of the age happened with the destruction of the second temple in 69 AD as prophesied in the book of the Apocalypse of John of Patmos.
The Old Testament had the concept of the Trinity - God walks in the garden, there are 3 who come to visit Lot in Sodom etc.