It's not enough to just "believe in Christ", because Man often lies to himself.

One must accept Christ as Lord for salvation and then labor in the process of sanctification, where they become progressively estranged from the sin of the world and the sin of their old selves.

Salvation without sanctification is called antinomianism, which means you can merely tell yourself you accept Christ and then return to your carnal self and remain unchanged.

Antinomianism is not Biblical.

"Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven." - KJV: Matthew: 7:21