In Heb 10:29
'Sanctified' is one of those words that you need to let the context dictate it's usage. The word literally means 'set apart' and it can apply (and often does) to true believers... but can also apply to objects, places and even people that aren't saved! Look at the following examples of what is said to be sanctified in the Bible:
Nation of Israel (Exodus 19:10)
Mount Sinai (Exodus 19:23)
Breast of the Ram (Exodus 29:27)
The Priests (Exodus 29:41)
The Tabernacle (Exodus 29:36)
In the New Testament we have the interesting verse in 1 Cor 7:14
"For the unbelieving husband has been sanctified through his wife, and the unbelieving wife has been sanctified through her believing husband. Otherwise your children would be unclean, but as it is, they are holy."
I can only take from this that where a believer is married to an unbeliever God sees the unbelieving partner (and any children) as 'set apart' or 'sanctified' because of the faith of their partner. Does this mean that the unbeliever is saved? No. But they are said to be sanctified or set apart. I think it would be fair to say that Judas was also 'set apart' as one of the 12 disciples to do the Lord's work, but not saved.
So Heb 10:29 is speaking of those that joined themselves to true believers in the church and in this sense were 'sanctified' or set apart. But they willfully left that going back on their initial belief and decision and went back to Judaism. They are apostates.
Concerning Heb 10:29, the well loved Bible teacher H.A Ironside says the following:
“There is an expression used in these verses that may still perplex and bewilder those who have not apprehended that profession is one thing, and possession another. … He who is sanctified by the one offering of Christ upon the cross, that is, by His precious blood, is perfected forever (Heb. 10:10, 14-19). But in this passage it is equally plain that one who counts the blood of the covenant, wherewith he was sanctified, an unholy thing, shall be forever lost. In order not to miss the true force of this for our souls, it is necessary that we give some attention to what we have already designated ‘positional sanctification.’ Of old all the people of Israel, and all who were associated with them, were set apart to God both on the night of the Passover and afterwards in the wilderness. But this did not necessarily imply a work of the Spirit in their souls. Many were doubtless in the blood-sprinkled houses that solemn night, when the destroying angel passed through to smite the unsheltered first-born, who had no real faith in God. Yet they were by the blood of the Lamb put in a place of blessing, a position where they shared in many hallowed privileges. So afterward with those who were under the cloud and passed through the sea, being baptized unto Moses in the cloud and in the sea. All were in the same position. All shared the same outward blessings. But the wilderness was the place of testing, and soon proved who were real and who were not. At the present time God has no special nation, to be allied to which is to come into a position of outward nearness to Him. But He has a people who have been redeemed to Himself out of all kindreds and tongues and peoples and nations, by the precious blood of the Lamb of God. All who ally themselves by profession with that company are outwardly among the blood-sheltered: in this sense they are sanctified by the blood of the covenant. That blood stands for Christianity, which in its very essence is the proclamation of salvation through Christ’s atoning death. To take the Christian place therefore is like entering the blood-sprinkled house. All who are real, who have judged themselves before God, and truely confided in his grace, will remain in that house. If they go out, it proves their unreality and such can find no other sacrifice for sins; for all the typical offerings are done away in Christ. These are they of whom the apostle John speaks so solemnly: ‘They went out from us, but they were not of us; for if they had been of us, they would have continued with us: but they went out that they might be made manifest that they were not all of us’ (1 John 2:19). As they were ever bereft of faith in the soul, they ‘went out,’ and thus did despite to the Spirit of grace, and counted the blood of the covenant, wherewith they were sanctified, an unholy thing. These sin wilfully, not in the sense of failing to walk uprightly merely, but as utterly abjuring, or apostatizing from, Christianity, after having become conversant with the glorious message it brings to lost men”