"And why call ye me, Lord, Lord, and do not the things which I say?"
Holy Bible, Luke 6:46
If you "talk the talk," you must also "walk the walk."
If you believe in God, you obey God's commandments. Believing in God is believing in who he is; part of who he is, is what he has said and done. God has said that those who disobey his commandments, and do not repent, will go to hell to suffer everlasting fire.
We know that, to be saved from hell, we must believe in Jesus Christ. But if you sin, you do not believe; because Jesus Christ said that sinners and wicked people will perish. Therefore, if you believed, you wouldn't sin; why would some act carelessly, and not care about being punished in hell for eternity? They wouldn't! That is only a result of ignorance, which stemmed from unbelief.