There was no concept of the "Holy Trinity" before 600 AD at the Vatican Council under Pope Gregory. It evolved as a compromise of ideas, rather than a universally understood truth.
Being good is exactly what it takes. Christ's last message to his disciples in His ministry on earth was
"Love one another, as I have loved you."
I have to believe this was His last message because it was crucial to our salvation. Not believing in Him as my Savior, not even believing in God. Love one another, and you shall be saved.