Many Scripture passages support the security of the believer (See the ATQ article, Should I believe in the Doctrine of Eternal Security?), while a few passages can be interpreted to imply the possibility of falling from grace. Overall, Scripture seems to support the view that genuine believers can backslide and experience discipline, but never fall from grace.
There is no way of knowing for sure whether a professing believer caught up in deep sin or apostasy was ever truly converted. So, practically speaking, when we consider our responsibility to God, it makes little difference whether we believe apostasy could lead to loss of salvation or whether someone who thinks they are a believer might be mistaken. Whichever view one takes, there is no grounds for living carelessly and irresponsibly.
Regardless of one’s view of eternal security, anyone self-satisfied and insensitive to sin in his life should be on guard. It is more important to pursue spiritual growth than to resolve the theoretical question of whether an apostate has never been saved or has lost his salvation.
Self-examination and re-dedication are important not so much to ensure that our salvation has not departed (or that we have truly been converted) as to keep our hearts tender and sensitive to the convicting work of the Holy Spirit. True spirituality isn’t based in fear but in our confidence in a heavenly Father who has already demonstrated His love for us (Isaiah 53:6; Romans 5:8; 1 John 4:9).
Whether we identify with Calvin or Arminius, our main concern should be relieving the fears of the insecure believer while confronting sinners (whether inside or outside the church) with their need for repentance.
Nevertheless the solid foundation of God stands, having this seal: “The Lord knows those who are His,” and, “Let everyone who names the name of Christ depart from iniquity” (2 Timothy 2:19 NKJV).
Practically speaking, eternal security isn’t worth arguing about.