What was Paul warning his readers in Hebrews 10?
So, couldn't it be understood that those being addressed sanctified themselves (middle voice), yet then became defiled? And it could not also refer to a future sanctification (aorist option)?
The First Covenant with "the children of Israel" is given in Exodus 19:5-6 and is a promise that Israel will be a kingdom of priests for the nations. We read this in Exodus 19:22 (in preparation for that future role):
"And let the priests also, which come near to the Lord, sanctify themselves [Hebrew reflexive case], lest the Lord break forth upon them."
So, a possible Scotto translation of Heb 10:29 (which we do not recommend) utilizing the NJKV translation:
MIDDLE VOICE
"Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he has sanctified himself a common thing, and insulted the Spirit of grace?"or
AORIST FUTURE OPTION
"Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he will be sanctified a common thing, and insulted the Spirit of grace?"or
MIDDLE VOICE, FUTURE TENSE
"Of how much worse punishment, do you suppose, will he be thought worthy who has trampled the Son of God underfoot, counted the blood of the covenant by which he will sanctify himself a common thing, and insulted the Spirit of grace?"
Paul is warning Hebrews not to neglect the salvation and implementation of the New Covenant (which was still future, see: Hebrews 8) that has been provided for them. The Old Covenant was passing away in anticipation of the New ("And what is becoming obsolete and growing old is ready to vanish away" 8:13b).
Just a thought.