How do the proponents of the idea that salvation once received cannot be lost explain Hebrews 6:4-6 and 10:26-29?

How do the proponents of the idea that salvation once received cannot be lost explain the following verses: Hebrews 6:4-6: > For [it is] impossible for those who were once enlightened, and have > tasted of the heavenly gift, and were made partakers of the Holy > Ghost, And have tasted the good word of God, and the powers of the > world to come, 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. and Hebrews 10:26-29: > For if we sin wilfully after that we have received the knowledge of > the truth, there remaineth no more sacrifice for sins, But a certain > fearful looking for of judgment and fiery indignation, which shall > devour the adversaries. He that despised Moses' law died without mercy > under two or three witnesses: Of how much sorer punishment, suppose > ye, shall he be thought worthy, who hath trodden under foot the Son of > God, and hath counted the blood of the covenant, wherewith he was > sanctified, an unholy thing, and hath done despite unto the Spirit of > grace? It seems to me that both places are about Christians who had once received salvation, but later lost it due to their change in mind or in faith.