The following is an excerpt from The MacArthur New Testament Commentary on Hebrews 10.

For if we go on sinning willfully after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain terrifying expectation of judgment, and the fury of a fire which will consume the adversaries.  (Hebrews 10:26-27)

Here is possibly the clearest and most concise scriptural definition of apostasy-receiving knowledge of the truth, that is, the gospel, but willfully remaining in sin. An apostate has seen and heard the truth—he knows it well—but he willfully rejects it.

Apostasy has two major characteristics: knowledge of the truth of the gospel and willful rejection of it.

Every apostate is an unbeliever, but not every unbeliever is an apostate. Many people have never had the opportunity to hear the gospel, even in part. They are sinful and, of course, do not believe in Christ, because they have never heard of Him or of His claims. An apostate, however, is well acquainted with the gospel. He knows more than enough to be saved.

The Greek language has two primary words that can be translated “knowledge.” Gnosis has to do with ordinary knowledge, and in the New Testament is often used for general spiritual knowledge. But epignosis, the word used in verse 26, denotes full knowledge, understanding, and discernment. In other words, the persons described here are those who have much more than a passing acquaintance with the gospel. They know it well. An apostate has all the information. He lacks nothing intellectually. He has epignosis. He is among those who have “once been enlightened, … tasted of the heavenly gift,” and even “been made partakers of the Holy Spirit” (Heb. 6:4).

An apostate can be bred only in the brilliant light of proximity to Christ. Apostates are not made in the absence, but in the presence, of Christ. They are bred almost without exception within the church, in the very midst of God’s people. It is possible for a person to read the Bible on his own, to see the gospel clearly, and then reject it—apart from direct association with Christians. But by and large, apostates come from within the church.

Eventually, sometimes even after years of pretense and self-deception, the unbeliever who acts like a believer finally falls away. He gives up, loses interest, and goes his own way. He returns to sinning willfully, with no more regard for the Lord’s way or the Lord’s people. To know God’s way, to study about it and hear about it, to identify with believers, and then turn away is to become apostate. The process of falling away may be gradual, but at some point a conscious decision is made to leave the way of God, and reject the saving grace of the Lord Jesus Christ.

Willfully (hekousios) carries the idea of deliberate intention that is habitual. The reference here is not to sins of ignorance or weakness, but to those that are planned out, determined, done with forethought. The difference between sins of ignorance and sinning willfully is much like the difference between involuntary manslaughter and first-degree murder. Hekousios is habitual. It not only is deliberate, but is an established way of thinking and believing. It is the permanent renunciation of the gospel, the permanent forsaking of God’s grace.

A believer may sometimes lapse into sin and stray from intimacy with the Lord and with His people. But, unless the Lord disciplines him and takes him to heaven, he will come back. He will be too much under conviction to stay away permanently. In the meanwhile, he will be robbed of joy and peace and of many other blessings.

We cannot always determine who is apostate and who is backsliding, and we should not try. We are not able to distinguish between a disobedient carnal believer and an apostate unbeliever. That is the Lord’s business. But there is a difference between the two, a very great difference. A person’s concern should be first of all that he himself is a true believer (2 Cor. 13:5) and then that he is a faithful believer. There are many calls to self-examination in the New Testament. Every time a believer comes to the Lord’s Table, he faces the reality or unreality of his salvation.