The word "apostasia" found in 2 Thessalonians 2:3 means ones who have defected from the truth of the Christian faith. It is a strong word implying revolt. It comes from a root word meaning to incite to revolt, to detest and depart. 

There Are A Number Of "Ifs" In The Bible Concerning Eternal Life.

if you hold fast that word - 1 Corinthians 15:2
if indeed you continue - Colossians 1:21-23
if we hold the beginning of our confidence steadfast to the end - Hebrews 3:14
if we disown him, he will also disown us; 2 Timothy 2:12 (NIV)

It says, "If we disown Him, He will also disown us." It is a simple truth that you must first be owned by Jesus before He can disown you. To be owned by Him means you are saved.

There Is A Point Where God Cuts Off Those Who Turn From Him.

If God did not spare natural Israel when they committed apostasy, what makes us think that God will spare us if we fall away. We must continue in His goodness otherwise we too will be cut off.

Hebrews 10:31,38-39 It is a fearful thing to fall into the hands of the living God.. Now the just shall live by faith; but if anyone draws back, my soul has no pleasure in him." But we are not of those who draw back to perdition, but of those who believe to the saving of the soul.

This drawing back "unto perdition" or waste and destruction is a possibility or it would not have been written to warn us. The word perdition indicates loss of well-being, not of being. The destruction then is everlasting without annihilation.

The Bible Warns It Is Possible to Fall Away Never to be Saved Again

There is a point where the once saved are completely reprobate in their minds and actions. Hebrews 6 speaks of the once Spirit-filled Christians falling away to never be saved again.

Hebrews 6:4-9 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.

The Greek word for fall away is parapipto, which means to fall aside or to apostatize (Strongs 3895).

This person has given up his birthright in the same way that Esaul did. This place is where backsliding has led to the ultimate place of disowning Christ as Paul warned Timothy of in 2 Timothy 2:12. 

This "falling away" does not mean to fall into sin. A person may fall into sin without "falling away". Many of God's people are backslidden and out of fellowship with God and His people yet they are still His children, i.e. their relationship has not been severed, only their fellowship. However, the one who "falls away" will, of course, go back into sin.

The person who commits apostasy becomes twice dead

When apostasy is committed they have committed the unpardonable sin of blaspheming the Holy Spirit by "crucifying to themselves the Son of God afresh". This means they consider themselves as dead to Christ once more. They are now twice dead as described in the epistle of Jude. When we were unsaved, we were dead to Christ and when we are saved, we are dead with Christ and dead to the world.