It is not - I can share the verses, but I suspect you have already heard them. I have stated before (not so succinctly), but the Bible is also self-authenticating. 

Jesus said  in John 10:27-28  “My sheep hear My voice, and I know them, and they follow Me; 28 and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand

It really is simple, if you do not believe you are not his, nor will you believe or follow.