Jaden... hi.... we don't have full understanding of sound doctrine the moment we get saved🙂
Correct, it takes learning and growing in Christ in a process we call sanctification. We start with the basic knowledge that God sent Jesus to die so that our sin can be forgiven, and we grow from there by study of the word, by prayer, by God's appointed teachers teaching us.
We may not come to the full knowledge until Christ comes. He will then teach us all things. If we have faith in him when comes again he will correct any error or misunderstanding that we have.
We must believe in our hearts and confess with our mouths that Jesus is Lord and that God raised him up.
Paul describes the Gospel as a mystery because it is beyond our ability to comprehend. I personally wonder why God would do many of the things he's has done. Why would he grant grace to a man as sinful as me? Yet he has revealed his grace to me and I believe and embrace him as my Lord, which is all by his mercy and Grace. I couldn't believe if he did not reveal himself to me, I couldn't embrace him as Lord except that he grant his grace to me.
This is a great and wonderful mystery.