Well, I've seen many people that have experienced "conversion", and a "change of life", and lived their whole lives for God...and then change their minds, committing themselves to unbelief and willing sin. You say "then they weren't really saved". But again...the point is, if those people weren't really saved, even though they were living a godly life at one point...where is your assurance? What makes you different from them? I believe saved people can fall. God will blot them out of the book of life.
Ezekiel 33:11 Say to them, �As surely as I live, declares the Sovereign Lord, I take no pleasure in the death of the wicked, but rather that they turn from their ways and live. Turn! Turn from your evil ways! Why will you die, O house of Israel?�
12�Therefore, son of man, say to your countrymen, �The righteousness of the righteous man will not save him when he disobeys, and the wickedness of the wicked man will not cause him to fall when he turns from it. The righteous man, if he sins, will not be allowed to live because of his former righteousness.� 13If I tell the righteous man that he will surely live, but then he trusts in his righteousness and does evil, none of the righteous things he has done will be remembered; he will die for the evil he has done. 14And if I say to the wicked man, �You will surely die,� but he then turns away from his sin and does what is just and right� 15if he gives back what he took in pledge for a loan, returns what he has stolen, follows the decrees that give life, and does no evil, he will surely live; he will not die. 16None of the sins he has committed will be remembered against him. He has done what is just and right; he will surely live.
17�Yet your countrymen say, �The way of the Lord is not just.� But it is their way that is not just. 18If a righteous man turns from his righteousness and does evil, he will die for it. 19And if a wicked man turns away from his wickedness and does what is just and right, he will live by doing so. 20Yet, O house of Israel, you say, �The way of the Lord is not just.� But I will judge each of you according to his own ways.�
Romans 11:19 Thou wilt say then, The branches were broken off, that I might be graffed in. 20 Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear: 21 For if God spared not the natural branches, take heed lest he also spare not thee. 22 Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off. 23 And they also, if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again.