The gospel is the power of God unto salvation
Romans 1:16 For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
Which was preached to the Jews first
Acts 3:26 Unto you (Jews) first God, having raised up his Son Jesus, sent him to bless you, in turning away every one of you from his iniquities.
And then to the Gentiles and so both here
Acts 20:21 Testifying both to the Jews, and also to the Greeks, repentance toward God, and faith toward our Lord Jesus Christ.
Saying,
Titus 2:14 Who gave himself for us, that he might redeem us from all iniquity, and purify unto himself a peculiar people, zealous of good works.
And so to purify unto himself a peculiar people, as Paul had said, "Let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of God" (in 2Cr 7:1, and the like in Ephes 4:24, James 1:21, 1 Thes 4:7, and in Heb 12:14 lastly) which tells us without which (holiness) no man shall see the Lord.
The context is the gospel of our salvation here as well, since it includes the words, "How shall we escape, if we neglect so great salvation" in respects to the word spoken by the Lord and them which heard him.
Heb 2:1 Therefore we ought to give the more earnest heed to the things which we have heard, lest at any time we should let them slip. For if the word spoken by angels was stedfast, and every transgression and disobedience received a just recompence of reward; How shall we escape, if we neglect so great salvation; which at the first began to be spoken by the Lord, and was confirmed unto us by them that heard him; God also bearing them witness, both with signs and wonders, and with divers miracles, and gifts of the Holy Ghost, according to his own will?
Again, the gospel of Christ it is the power of God unto salvation to every one that believeth, the same is mentioned here.
Hebrews 4:1-3 Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it. For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it.For we which have believed do enter into rest, as he said, As I have sworn in my wrath, if they shall enter into my rest: although the works were finished from the foundation of the world.
And again here it says, that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned.
Hebrews 6:4-9 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame. For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God: But that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned.
And we know this because it ends with...
(9) But, beloved, we are persuaded better things of you, and things that accompany salvation, though we thus speak.
Thorns and briers is rejected, these are not fruit
Which is what should also accompany our common salvation.
Since the gospel bringeth forth fruit as Paul tells us (in Col 1:6) as it doth also in you, since the day ye heard of it, and "knew the grace of God in truth"
Whereas Jude writes concerning our common salvation, to warn of ungodly men which turn the grace of our God into lasciviousness.
Jude 1:3-5 Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints. 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. I will therefore put you in remembrance, though ye once knew this, how that the Lord, having saved the people out of the land of Egypt, afterward destroyed them that believed not.
Edit: typos