If you accepted Jesus Christ as your personal savior, you become His adopted child. Even through at times we may sin, it does not cause us to lose our status in His kingdom ( see John 8:31~34), He can never disowned us. But since God did give us free will, Luke chap. 12:10 does warn us not to blaspheme against the Holy Spirit. If we denied the Christ, it can be forgiven. But denying the Holy Spirit can not be forgiven. See Hebrews 6:4~7. Basically, once we belong to Christ, there is nothing we can do to lose our salvation from Him, His grace continues everyday. But since God gave us free will, one could reject salvation in which he could never get it back cause that would mean Jesus Christ would have to die again on the cross in which He would not do cause He had already defeated death and given us eternal life. So if one rejects salvation, his heart will become harden towards God. This is a very quick answer for a very deep question. I hope this helped.