Hebrews 3:12 "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God."
Unbelief is the sin that separates men from God. Here are but a few examples found throughout the word. Adam did not believe God. Therefore he was cast out of the presence of God. Cain did not believe that the only way he could approach God was in the blood of a substitute. Therefore his works were rejected. During Noah's day all flesh had corrupted his way upon the earth and they would not enter into Christ, the true ark. By their unbelief they perished under the flood of God's justice.
Consider, Pharaoh, who hardened his heart against God and received the plagues of the Sovereign. Think of Pharaoh's Egyptian army. Upon realizing the particular, electing grace of God, rather than fleeing to the Lord for mercy, they cried, Let us flee from the face of Israel; for the Lord fighteth for them against the Egyptians. Because of their unbelief God drowned them in the Red Sea.
Scripture says that the gospel was preached unto all of Israel as well as us today, but the word preached did not profit them, not being mixed with faith in them that heard it. Though not all of Israel perished in unbelief, consider those who did. They perverted the gospel of God, statutes like circumcision, the Passover, and the Sabbath day. They thought righteousness came by obedience to these memorials. In their unbelief they never considered Christ whom the ceremonies typified.
By the grace of God his people believe (then and now) that God gave circumcision as a token of the promise of salvation through Christ Jesus. It is a picture of God changing a man by writing His law in our heart. His people believe (then and now) that God instituted the Passover as a celebration of freedom from the bondage of sin found only in Christ Jesus, the true Lamb of God. In Moses' day the Lord's people believed that the Lord Jesus Christ would come. Today we believe that He has come. The believer's hope is that He of whom the ceremonies pointed put an end to that law, provided us a perfect righteousness and is now seated with God until He return for what is rightfully His. In Christ our Sabbath there remaineth therefore a rest to the people of God. For he that is entered into his rest, he also hath ceased from his own works, as God did from his. Let us labor therefore to enter into that rest, lest any man fall after the same example of unbelief.