Romans 10:9-10: “(T)hat if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved; for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation.”
It isn’t enough to believe in one’s heart that Christ is the sacrifice for our sins, but we have to be willing to confess this before men. I don’t believe this to be a one time thing, either; we must be willing to confess Him whenever the situation presents, and not deny that we believe and hope in Him. The situation with Peter’s denial shows that we can repent of moments of weakness in this regard, but a continual denial of Him will lead to our being lost, for, ““Therefore whoever confesses Me before men, him I will also confess before My Father who is in heaven. But whoever denies Me before men, him I will also deny before My Father who is in heaven,” Matthew 10:32-33.