While a person today is likely saved when he accepts Christ as His Savior with as little faith as the thief on the Cross, the same saved soul who stumbles upon the Trinity, rejecting the Trinity for a false doctrine, falls out of fellowship and fails to advance spiritually.
The Christian walk is a spiritual walk.
Did St. Paul fail to advance? Yet he never knew the doctrine of the Trinity.
The finest Christians I have known do not believe in the man made Trinity doctrine. Either you are repeating something you have been taught or your acquaintances are limited.