How does apostasy happen?

Apostasy happens slowly. That's how it happens.

Let's take a look at 1st Timothy:

"Now the Spirit expressly says that in later times some will depart from the faith by giving heed to deceitful spirits and doctrines of demons, through the pretensions of liars whose consciences are seared, who forbid marriage and enjoin abstinence from foods which God created to be received with thanksgiving by those who believe and know the truth. For everything created by God is good, and nothing is to be rejected if it is received with thanksgiving; for then it is consecrated by the word of God and prayer." (1 Timothy 4:1-5 RSV).

The Spirit says, so this is a true saying. He also says it expressly. Other translations use the world clearly or explicitly.The Spirit wished to impart a clear and pointed teaching, not in metaphors or parables or symbols but in clear words, for emphasis. He expressly says some will depart from the faith. Not that they ever were true Christians (though many would have thought they were) but that they would leave the doctrines that undergird our faith and give expression to it. How would they do this? The Spirit continues: they gave heed to deceitful spirits and doctrines of demons.

They gave heed. This means that they yielded some small part of their mind or their heart to some small portion of untruth, even for a moment. But that is all it takes. Remember the first words of satan in the bible, Genesis 3: "Hath God said?" This is a direct attack on His word, and Eve gave heed to it. You know the rest.

"But understand this, that in the last days there will come times of stress. For men will be lovers of self, lovers of money, proud, arrogant, abusive, disobedient to their parents, ungrateful, unholy, inhuman, implacable, slanderers, profligates, fierce, haters of good, treacherous, reckless, swollen with conceit, lovers of pleasure rather than lovers of God, holding the form of religion but denying the power of it. Avoid such people." (2 Timothy 3:1-5 RSV)

These who fall away from the faith have a form of religion. The outward appearance is one that fools people -for a while- that they are true believers.

Paul wrote about Demas three times. Demas was a co-worker alongside Paul for many years. Demas was with many wonderful men of the faith: Luke, Epahras, others. He had traveled, witnessed, prayed, served for many years.

We see Paul mention him in Colossians-
1. Our dear friend Luke, the doctor, and Demas send greetings. (Colossians 4:14)

We see Paul mention him in Philemon: 
2. Mark, Aristarchus, Demas, and Luke, my co-workers. (Philemon 1:24) 

We see Paul mention him in 2 Timothy:
3. for Demas, because he loved this world, has deserted me and has gone to Thessalonica. (2 Timothy 4:10)

Worker, worker, gone. That's it. He's gone. How? Why? Because Demas loved the world. Obviously the Father had not been in him, because you can't have the world AND the Father in you. (1 John 2:15). One or the other will win out, and it will be whichever one you give heed to.

So did Demas leave suddenly? Physically, probably yes, his departure was probably abrupt. But how long had the apostasy been in him? All along. You sink slowly. Let's look at an incident that I believe illustrates this. Peter was not apostate, but the metaphor here is interesting because of the physics of it. Remember when the storm was tossing the boat, and Jesus walked over to the men in the boat on top of the water? Peter asked the Lord, if that is really you, then I will walk on water. And Peter got out of the boat, and for a minute he walked on water too. But when he noticed the stormy circumstances and took his eyes off Jesus, is when he lost it. Most people teach that passage as an illustration of how we have to keep our eyes on Jesus, and it is true, we do. But I'm interested here in the phrase, began to sink:

"But when he saw the wind, he was afraid, and beginning to sink he cried out, “Lord, save me." (Matthew 14:30).

Every translation records the same, "began to sink." Think about it. Peter was a burly guy: using his body and his hands to make a living fishing. He had muscles. He was probably 175-200 pounds. Solid muscle. If you toss a 200 pound boulder in the water, does it begin to sink? Or does it sink? Plunk, gone? The latter, it just goes under. But here the bible records Peter sinking incrementally.

That's what happens in apostasy, you slowly sink under the waves of sin, until you're completely covered. Apostasy is an incremental departure. It is like in Hebrews where it says:

"Therefore we must pay much closer attention to what we have heard, lest we drift away from it. (Hebrews 2:1)

The Greek word pararrhyéō ("drift away from") only occurs in Heb 2:1 where it refers to going spiritually adrift – "sinning by slipping away" (from God's anchor). 3901 /pararrhyéō ("gradually drift away") means to "lapse" into spiritual defeat, describing how we slowly move away from our moorings in Christ. (source)

How can you make sure you love Christ and not the world, that apostasy has not crept in, that you are not drifting away? Of course our wonderful bible has the answer:

"Examine yourselves, to see whether you are in the faith. Test yourselves. Or do you not realize this about yourselves, that Jesus Christ is in you?—unless indeed you fail to meet the test!" (2 Cor 13:5)

So...how do we examine ourselves? Here is one page using scripture that helps guide us through the common tests:

Are You A Christian?
TEST 1: Gospel Belief and Confession of Faith
2) Born Again / Sonship / Correction;
3) Repentance / Deliverance from Sin;
4) Good Works by Grace, and
5) The Fruit of the Spirit. 

Don't sink, don't leave, don't drift. Examine yourself and stay in the faith!