Can Sins Be Forgiven after Baptism?

What are we to make of the Biblical book of Hebrews 6:4-6 and 10:26-27? They teach that "it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 And have tasted the good word of God, and the powers of the world to come,6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.  For if we sin wilfully after that