Note that Heb. 10:26 (“For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins”) does not refer to those who are saved, but to those who “have received the knowledge of the truth”.
Just having heard the Gospel (having “received the knowledge of the truth”) does not mean one is saved, as Christ pointed out in the parable of the sower (Matt. 13:18-23), some who had heard “the word of the kingdom” had not understood the Gospel truth to begin with, some understood, but fell away from the faith for various reasons, and some did understand it, and bore the fruit of it (did their best to obey Christ’s teachings) and endured in the faith. As Christ said in Matt. 10:22, "And ye shall be hated of all men for my name’s sake: but
he that endureth to the end shall be saved."
Remember that Christ said, “many be called, few are chosen” (Matt. 22:14). Heb. 10:26 refers to those who had been called, but were not chosen; those who thought professing to be a believer gave them a license to sin, those like Jude 1:4 refers to who had crept into the congregation and had turned the “grace of our God into lasciviousness” (i.e. licentiousness, lawlessness, immorality), those who had “counted the blood of the covenant, wherewith he was sanctified, an unholy thing” (Heb. 10:29) by attempting to use the holy sacrifice of Christ as an excuse to sin and to act wickedly against their fellow man, their “neighbor”. Those who “wilfully sin” are not real believers; they were not ever saved, and therefore had never received the Holy Spirit.
Heb 10:22 Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water.
Heb 10:23 Let us hold fast the profession of our faith without wavering; (for he is faithful that promised

Heb 10:24 And let us consider one another to provoke unto love and to good works:
Heb 10:25 Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.
Heb 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,
Heb 10:27 But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries.
Heb 10:28 He that despised Moses’ law died without mercy under two or three witnesses:
Heb 10:29 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 [have insulted] the Spirit of grace?
[KJV Bible]]
Christ said:
Mat 13:18 Hear ye therefore the parable of the sower.
Mat 13:19 When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side.
Mat 13:20 But he that received the seed into stony places, the same is he that heareth the word, and anon with joy receiveth it;
Mat 13:21 Yet hath he not root in himself, but dureth for a while: for when tribulation or persecution ariseth because of the word, by and by he is offended.
Mat 13:22 He also that received seed among the thorns is he that heareth the word; and the care of this world, and the deceitfulness of riches, choke the word, and he becometh unfruitful.
Mat 13:23 But he that received seed into the good ground is he that heareth the word, and understandeth it; which also beareth fruit, and bringeth forth, some an hundredfold, some sixty, some thirty.
[KJV Bible]
Jude 1:4, “For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.”)
[KJV Bible]