Apostasy means "to fall away". An apostate is one who has heard the gospel, professed faith, but then eventually abandons (falls away) from the faith indicating that they never actually professed true faith. The last days are perilous times precisely because they are days of APOSTASY! (see 2 Tim. 3:1-5)

At this point the gospel has gone out throughout the whole world (generally speaking). Of course, there are many that have not yet heard, but the gospel has been taken to every nation in the world. And in places such as Europe where the gospel has been largely shared in the past what has followed has been widespread apostasy. Today Europe is essentially a shell of apostasy. And so is the United States. Praise God He always has a REMNANT! (see Rom. 11:5)

Apostates have heard and turned from the truth. Apostates have known the truth intellectually. Apostasy is an especially serious sin precisely because these people have known the truth. Peter speaks to the nature of apostasy in 2 Pet. 2.

2 Peter 2:20–22 (NKJV)
20 For if, after they have escaped the pollutions of the world through the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and overcome, the latter end is worse for them than the beginning.
21 For it would have been better for them not to have known the way of righteousness, than having known it, to turn from the holy commandment delivered to them.
22 But it has happened to them according to the true proverb: “A dog returns to his own vomit,” and, “a sow, having washed, to her wallowing in the mire.”

These people have come to the place of “escape”, but they have not really escaped, as the text goes on to show. They had "the knowledge of the Lord and Savior Jesus Christ" but all to no avail. In order to escape the power of sin one must know about the Deliverer. One must have this knowledge - the knowledge of the truth (cf. 1 Tim. 2:4). These people have had it. They have intellectually come to know this truth. Apostates have KNOWLEDGE but they are not yet truly saved. In fact, they may have somewhat intimate knowledge which is what makes them so very accountable (cf. Heb. 6:4-6, 10:26-29). These people know the facts of the gospel very well. They have known the language of “Lord and Savior” very well. There is no sin more scarier than this! Jesus indicated that MANY will say "Lord, Lord" on judgment day but will be shown the door to HELL (cf. Matt. 7:21-23).

Apostates may have had a temporary reformation. One can make the flesh behave for a while. There may have been some outward cleaning up of the life in which they may have lived the life of a moralist, but there was no real internal transformation. There was no real change of heart - no genuine repentance. The washed sow remains a mud loving sow by nature!

The "holy commandment" they turned from refers to the authoritative, apostolic gospel message - the message of Christ as Lord and Savior. The whole of the Christian gospel can be summed up in the PERSON and WORK of Christ in the language of “Lord and Savior” as Peter succinctly puts it in 2 Pet. 2:20. This is not a message one can take or leave. This is a HOLY COMMANDMENT! This is not a mere suggestion! God demands that we bow before the Lordship of Christ, and in humility receive Christ as our personal Lord and Savior! (see Acts 17:30-31) And when this is done in TRUTH it is a life changing reality! Apostates have gone through the motions, but it has not been real in their heart.

Peter says it would have been better for them not to even have known the truth than having known it to turn away. Jesus said of Judas, it would have been better never for him to have been born (cf. Mt. 26:24). To know the truth is to be responsible for it.

Peter quoted from Proverbs 26:11 showing that the issue is an unchanged nature. He says, “A dog returns to his own vomit”. The disgusting fact is that a dog will return to its own vomit to sniff, to lick, and to eat. No matter how refined a dog may be, no matter how well trained and house broken, a dog is still a dog by nature, and it will continue to do disgusting things no matter what. The nature of the dog is UNCHANGED! And so it is with APOSTATES! They have known the truth and yet remained unchanged by it!

The false prophet Balaam is an example of glaring apostasy. He sinned with his eyes "wide open". It is ironic how God forced him to prophesy that which he didn't want to do. He said his utterance was the "utterance of a man whose eyes are opened" (Num. 24:3). He saw the truth but his heart was not there.

Numbers 24:4 (NKJV)
4 The utterance of him who hears the words of God, who sees the vision of the Almighty, who falls down, with eyes wide open:

Peter in the context of 2 Peter 2 uses Balaam as a key example of APOSTASY (see 2 Pet. 2:15-16). The sin of apostasy is so very serious because properly understood these people sin with their "eyes wide open". They willfully reject the truth! (see 2 Pet. 3:5) There is no remedy for full blown apostasy. The warnings in Scripture are severe. Hebrews 3:7-8 warns: "Today, if you will hear His voice, do not harden your hearts".

If you are looking for a "sign" of the last days related to the Church Age the TREND that the New Testament presents is that of GROWING APOSTASY! Yes, MANY have HEARD, and MANY have PROFESSED, but the issue is how many have truly appropriated the truth of Christ as LORD AND SAVIOR! A true saving faith is life changing and it continues!

1 Timothy 4:1 (NKJV)
1 Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons,

2 Thessalonians 2:11–12 (NKJV)
11 And for this reason God will send them strong delusion, that they should believe the lie,
12 that they all may be condemned who did not believe the truth but had pleasure in unrighteousness.

2 Timothy 3:13 (NKJV)
13 But evil men and impostors will grow worse and worse, deceiving and being deceived.

Hebrews 10:26–27 (NKJV)
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.

1 John 2:19 (NKJV)
19 They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.