Hebrews 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins,
This verse is about the Mass. If you read Heb 10:25-31, you’ll see that God is offended by those who do not come to the Assembly, or the Mass. Basically explains why it is a mortal sin to skip the Mass.
1 John 3:9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
When I read St. John’s writings, I keep in mind that this is the beloved Disciple. St. John obviously had a capacity for love of God that has rarely been equaled by any other mortal. So, let’s look at the context:
3 And every man that hath this hope in him purifieth himself, even as he is pure.
He is talking about people who have put their entire hope in God, so much so, that they will do their best to remain pure.
4 Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
he makes sure that we know that sin is any transgression of the Ten Commandments. Disobeying God, is transgression of the first Commandment. Therefore, at this point, it is completely in line with Heb 10:25-31.
5 And ye know that he was manifested to take away our sins; and in him is no sin.
God became man to take away our sins and as a man, He did not sin.
6 Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
Those who abide in Christ, will not sin. But those who do sin, do so because they don’t know Him.
Interestingly, this ties us back to the Mass. Because Jesus said:
John 6:56 He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him.
The word dwell, is also translated, abide. So, it is those who attend the Mass and participate in the Eucharist, who abide in Christ. Those who don’t, sin in not doing so. And they don’t attend the Mass because they don’t know Christ.
7 Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous.
8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
Those who, after knowing Christ, have chosen to remain in sin, are the offspring of Satan.
9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
But those who are truly born of God and abide in Christ, will not commit sin.
I hope that helps.