Reading the next 2 verses clears up this misunderstanding. But, aside from all the contextual considerations, here on Hebrews 10, we find in 1 John 1:7-9, that John says positively what Hebrews here says negatively. John says that those Christians who walk in the light, that is repent and confess their sins, will have the cleansing of Christ's sacrifice. Obviously the word "if" in John's statement means that the opposite is true of those Christians who are not walking in the light, that is, not repenting and confessing their sins. What is the opposite? That they will not have the cleansing of Christ's sacrifice. And we know that John is speaking exclusively of Christians, since his letter is to the churches and he says "we." Now, with that in mind let us look at the next 2 verses in Hebrews 10:
(Heb 10:28-29) He that despised Moses' law died without mercy under two or three witnesses: {29} 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?
Also, Heb 10:39 clears this up contextually, saying "But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul."
He is refering in verse 26 to those who had received Christ's sacrifice but later rejected it, not in word, but in action, that is they rejected it by refusing to live in the manner that one who accepts it must live in. Now, he compares these who sin against Christ's sacrifice to those who sinned against sacrifices under Moses' Law, saying they are worthy of worse punishment, since they have stomped all over Christ, &c. Those who accept the sacrifice of Christ and become Christians but then persist in wilfull sin have drawn back to perdition, as the apostle terms it in vs 39, and have become essentially unbelievers. Yes, they believe in Jesus, but they don't believe Jesus. They are like Moses in his striking the rock when God said to speak to it, to which God replied "Because you did not believe Me, to hallow Me in the eyes of the children of Israel, therefore you shall not bring this assembly into the land which I have given them." (Num 20:12) And their punishment is the same as that of Moses - they will not enter the promised land.