He wasn't a genuine believer:
and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing; or "common thing"; putting it upon a level with the blood of a bullock, or at most counting it איך דכלנש, "as that of another man"; as the Syriac version renders it; yea, reckoning it as unclean and abominable, as the blood of a very wicked man: this is aggravated by its being "the blood of the covenant"; of the covenant of grace, because that is ratified and confirmed by it, and the blessings of it come through it; and from sanctification by it: either of the person, the apostate himself, who was sanctified or separated from others by a visible profession of religion; having given himself up to a church, to walk with it in the ordinances of the Gospel; and having submitted to baptism, and partook of the Lord's supper, and drank of the cup, "the blood of the New Testament", or "covenant": though he did not spiritually discern the body and blood of Christ in the ordinance, but counted the bread and wine, the symbols of them, as common things; or who professed himself, and was looked upon by others, to be truly sanctified by the Spirit, and to be justified by the blood of Christ, though he was not really so: or rather the Son of God himself is meant, who was sanctified, set apart, hallowed, and consecrated, as Aaron and his sons were sanctified by the sacrifices of slain beasts, to minister in the priest's office: so Christ, when he had offered himself, and shed his precious blood, by which the covenant of grace was ratified, by the same blood he was brought again from the dead, and declared to be the Son of God with power; and being set down at God's right hand, he ever lives to make intercession, which is the other part of his priestly office he is sanctified by his own blood to accomplish. This clause, "wherewith he was sanctified", is left out in the Alexandrian copy:" - John Gill