Hebrews 10:26-31
²⁶ For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, ²⁷ but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries. ²⁸ Anyone who has rejected Moses’ law dies without mercy on the testimony of two or three witnesses. ²⁹ 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 was sanctified a common thing, and insulted the Spirit of grace? ³⁰ For we know Him who said, “Vengeance is Mine, I will repay,”says the Lord. And again, “The Lord will judge His people.” ³¹ It is a fearful thing to fall into the hands of the living God.

When I started studying scripture in depth, this passage was a huge eye opener!!
I was taught the idea of “Once-Saved-Always-Saved” for thirty years. This one bit of scripture tells me that is not true, along with this one as well:
John 3:36
He who trusts in the Son has eternal life. He who does not obey the Son will not see life, but the wrath of God remains on him.

This is serious business, right?
Once-saved-always-saved allows us to go about our business without  a great deal of concern [just as Satan wants it!!].
But these scriptures should cause us great concern!!
If we are paying attention, studying scripture, praying for understanding — we should understand these (and other) warnings.  And even more importantly, we should know if we are violating them or not.

I try to share these ideas with many friends in mainstream church. My rejection rate runs somewhere near 100%.
Should I walk away and give up?
Nope.
Not if I love my neighbor as myself.
So I keep trying.

With that in mind, let’s take a look at these tragic warnings.
If continued, willful sin means we can expect judgement instead of salvation, and that judgement will consist of fiery destruction, then we need to know what sin is.
No really.
We absolutely have to know what sin is!!
We cannot heed these warnings without this knowledge.

How should man define sin?
Is it up to us to define it, and then act accordingly, to the best of our ability? What if we decide what is sin, and what is not sin, and we get it wrong?  Do we run the risk of the consequences outlined in Hebrews 10:26-31?
If so, salvation is at stake.
I’m not willing to try to interpret what sin is, since my salvation is on the line. I’d rather let scripture define sin.
That way, I know I’ll understand properly, and can avoid the Hebrews 10:26-31 consequences of fiery judgement [not my words, but what scripture says].

1John 3:4
[TLV] Everyone practicing sin also practices lawlessness—indeed, sin is lawlessness.

[NKJV] Whoever commits sin also commits lawlessness, and sin is lawlessness. 

[KJV] Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.

[ESV] Everyone who makes a practice of sinning also practices lawlessness; (M)sin is lawlessness. 

[CJB] Everyone who keeps sinning is violating Torah — indeed, sin is violation of Torah.

[ASV] Every one that doeth sin doeth also lawlessness; and sin is lawlessness. 

[1599 Geneva] Whosoever commiteth sin, transgresseth also the Law: for sin is the transgression of the Law.

Various bible translations are in agreement.
Sin is the violation of the law given in Torah.
If we go about our lives, violating these concepts without concern, say hello to fiery indignation and destruction.

Really.
Not kidding.
Go back and read Hebrews 10:26-31 again.

If what I’m presenting here is foreign to you, and isn’t what you’ve been taught your whole life, I get it.
I also was taught other things my whole life.
But now I study scripture.
I will not trust my salvation to a guy who went to college and learned what other men teach about the bible.
I will be responsible for myself.
I will be responsible for understanding salvation in scripture.
I encourage you to do the same.
If you are wondering why I might doubt our educated religious leaders, here are a couple of good reasons:

Jeremiah 16:19-20
¹⁹ O Lord, my strength and my fortress,
My refuge in the day of affliction,
The Gentiles shall come to You
From the ends of the earth and say,
“Surely our fathers have inherited lies,
Worthlessness and unprofitable things.”
²⁰ Will a man make gods for himself,
Which are not gods?

Matthew 23 (Yeshua Messiah speaking to the educated religious leaders of His day):
Woe to the Scribes and Pharisees
23 Then Jesus spoke to the multitudes and to His disciples, ² saying: “The scribes and the Pharisees sit in Moses’ seat. ³ Therefore whatever they tell you to observe, that observe and do, but do not do according to their works; for they say, and do not do. ⁴ For they bind heavy burdens, hard to bear, and lay them on men’s shoulders; but they themselves will not move them with one of their fingers. ⁵ But all their works they do to be seen by men. They make their phylacteries broad and enlarge the borders of their garments. ⁶ They love the best places at feasts, the best seats in the synagogues, ⁷ greetings in the marketplaces, and to be called by men, ‘Rabbi, Rabbi.’ ⁸ But you, do not be called ‘Rabbi’; for One is your Teacher, the Christ, and you are all brethren. ⁹ Do not call anyone on earth your father; for One is your Father, He who is in heaven. ¹⁰ And do not be called teachers; for One is your Teacher, the Christ. ¹¹ But he who is greatest among you shall be your servant. ¹² And whoever exalts himself will be humbled, and he who humbles himself will be exalted.

¹³ “But woe to you, scribes and Pharisees, hypocrites! For you shut up the kingdom of heaven against men; for you neither go in yourselves,nor do you allow those who are entering to go in. ¹⁴ Woe to you, scribes and Pharisees, hypocrites! For you devour widows’ houses, and for a pretense make long prayers. Therefore you will receive greater condemnation.

¹⁵ “Woe to you, scribes and Pharisees, hypocrites! For you travel land and sea to win one proselyte, and when he is won, you make him twice as much a son of hell as yourselves.

¹⁶ “Woe to you, blind guides, who say, ‘Whoever swears by the temple, it is nothing; but whoever swears by the gold of the temple, he is obliged to perform it.’ ¹⁷ Fools and blind! For which is greater, the gold or the temple that sanctifies^[d] the gold? ¹⁸ And, ‘Whoever swears by the altar, it is nothing; but whoever swears by the gift that is on it, he is obliged to perform it.’ ¹⁹ Fools and blind! For which is greater, the gift or the altar that sanctifies the gift? ²⁰ Therefore he who swears by the altar, swears by it and by all things on it. ²¹ He who swears by the temple, swears by it and by Him who dwells in it. ²² And he who swears by heaven, swears by the throne of God and by Him who sits on it.

²³ “Woe to you, scribes and Pharisees, hypocrites! For you pay tithe of mint and anise and cummin, and have neglected the weightier mattersof the law: justice and mercy and faith. These you ought to have done, without leaving the others undone. ²⁴ Blind guides, who strain out a gnat and swallow a camel!

²⁵ “Woe to you, scribes and Pharisees, hypocrites! For you cleanse the outside of the cup and dish, but inside they are full of extortion and self-indulgence. ²⁶ Blind Pharisee, first cleanse the inside of the cup and dish, that the outside of them may be clean also.

²⁷ “Woe to you, scribes and Pharisees, hypocrites! For you are like whitewashed tombs which indeed appear beautiful outwardly, but inside are full of dead men’s bones and all uncleanness. ²⁸ Even so you also outwardly appear righteous to men, but inside you are full of hypocrisy and lawlessness.

²⁹ “Woe to you, scribes and Pharisees, hypocrites! Because you build the tombs of the prophets and adorn the monuments of the righteous, ³⁰ and say, ‘If we had lived in the days of our fathers, we would not have been partakers with them in the blood of the prophets.’

³¹ “Therefore you are witnesses against yourselves that you are sons of those who murdered the prophets. ³² Fill up, then, the measure of your fathers’ guilt. ³³ Serpents, brood of vipers! How can you escape the condemnation of hell? ³⁴ Therefore, indeed, I send you prophets, wise men, and scribes: some of them you will kill and crucify, and some of them you will scourge in your synagogues and persecute from city to city, ³⁵ that on you may come all the righteous blood shed on the earth, from the blood of righteous Abel to the blood of Zechariah, son of Berechiah, whom you murdered between the temple and the altar.³⁶ Assuredly, I say to you, all these things will come upon this generation.