You fail to understand that the promise of salvation is conditional. When one becomes a believer, one receives a conditional promise of eternal life - one is not immediately granted an irrevocable ticket to Heaven - the ticket you receive on coming to faith is provisional and can be revoked. You are granted eternal life only after you die and are judged worthy by Christ.
Hebrews clearly states that believers can fall away and have their ticket to Heaven revoked and torn up:
4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.