Well I will give you my answer,  if you are really a child of God, saved that is, you are saved forever, John 3:16 tells you that, so if you sway because of getting away from the Lord or sin you can go to the Lord and ask forgivness and he will forgive you, He already has forgiven you He just wants you to still ask for forgivness.  Now if you have never really trusted in the saving power of the Lord you will have to accept Christ into your heart, ask forgiveness of your sins and know and truly believe that Jesus died for you and thank Him for salvation. If that person believes in God but not the SON then I would say he is not a true Christian or believer, he is only believing in his head that there is a God. and yes the only way to God is throught the Lord Jesus Christ.  That is why he died on the cross to save us He is our mediator between us and God. So I hope this helps you a little.  If you need more help in explaining things to that person you can purchase a bible from a christian book store (a New Testament) that has the plan of salvation written in red through the gospels, if you can't find one let me know and I will see what I can get for you here.  Love,