Not all have this attitude "before they left." Some have it even while in the church. But yes, I agree that it's often a reflection of their relationship with Christ before they left the church. The New Testament says it is proof that they never were truly one of us to begin with.

Excellent points, Shane. However I've found very few if any souls who are striving to avoid God's requirements, persuaded by the best of "proofs" in favor of His requirements. Could it be that those who wish to disprove the clear Word are listening to the same sophistry to which Adam and Eve listened, to the detriment of all those that follow after them.

Those who claim salvation by grace without seeing the necessity for fulfilling the necessary requirements of a perfect creation, are legalists in the highest sense of the word, because they feel they must do all the Law requires for salvation. Their solution is to destroy the Law rather than asking the God of the universe, of Whom there is nothing too hard for Him, to fashion within them all the requirements necessary for eternal life, which would include writing the Law on the fleshy tables of their heart.
Their solution in theory removes the need for a new creation and allows the most depraved continuum to have confidence in a faith based on ropes of sand. But then only a believer in the God Who can do anything, can believe in that type of reasoning.

"But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned." 1 Corinthians 2:14 KJV

That seems one good reason to sow the seeds of Truth and let God take care of the growth. Maybe it's possible sometimes in an arena of debate, but I find it highly unlikely since each person debating is far more interested in their own teaching than they are in their own learning, or so it seems to me.

"...Each person must be fully convinced in his own mind." Romans 14:5 NASB
Blessings!!

May I suggest the main reason for this attitude is neglecting the Word in favor of reading the Spirit of Prophecy. Passing judgement on the Word from another source is exactly what was warned against by EGW. We are promised the Holy Spirit through contact with the Word.

"If ye then, being evil, know how to give good gifts unto your children: how much more shall your heavenly Father give the Holy Spirit to them that ask him?"
Luke 11:13 KJV

"Howbeit when he, the Spirit of truth, is come, he will guide you into all truth" John 16:13 KJV

The Spirit of Prophecy is there primarily to verify what is already written in the Word. If it seems to disagree with the Word, we need to find from the Word the correct correlation between the Word and the servant sent to make the Word more clear.
Regards!!