Nessa: Theres no reason to believe they weren't trusting and believing in God for salvation.
Nessa, look at the described behaviors of those Jesus condemns. He knew their hearts and minds - and not JUST their outward behaviors - which is all we can see. What we don't see in Scripture is faithful people of long term simply falling away. Yes, people hung with the Apostles that were all caught up in being part of groups of Believers - for awhile. Are we to believe God makes a mistake - that He saves people that He knows aren't sincere? Or that He saves people merely hoping THEY will remain faithful IN THEIR OWN POWER???!!! How does ANYONE remain faithful, if not for Christ and God's Spirit within? Are we to think that God wants all to come to salvation, that He saves people, only to later abandon them to their own fallible flesh? We did NOT come to the Lord in our own power and WE cannot SUSTAIN what HE put within us in OUR own power. The fact that you realize that you cannot makes you doubt. But I would be that what you doubt is not GOD'S faithfulness, but your own ability to stay faithful in YOUR power. And we ALL should doubt that - because we know ourselves - we KNOW that unless the Lord sustains our salvation, we are in MAJOR trouble.
AGAIN, remember Paul's CONFIDENCE, that if dependent upon man, was exceptionally foolish:
"And I am SURE of this, that HE who began a good work in you WILL bring it to completion at the day of Jesus Christ."
Our salvation was never dependent upon God looking into the future as to our persevering in our own power - NO, He looked into our futures seeing HIS faithfulness and sustaining power. Again, when we see people who definitely fell away, the could never have been saved to begin with!
Nessa, I know this struggle, oh so well, because I know myself so well. I KNOW that I am not dependable to do what I KNOW I cannot do - remain faithful in my own power. In God's power - YES, all things, including staying faithful, are DEFINITELY, not only possible, but are a certainty! But we so often go on our feelings related to 1) knowing so well our own sinful natures and past track record AND 2) the devil constantly, accusingly whispers in our ears - "You KNOW yourself, you sinful, pathetic human - you are delusional that you will find final acceptance by the Lord Most High." But God tells us that He sees us not as we are in the flesh, but as we one day SHALL be! The best, most spiritual Christian amongst us is, to God, a sinful man just like the next. And might I remind you, that God used sinful, weak, unpredictable, unreliable, often cowardly, clueless men (and women) throughout Scripture to change the world and reveal His message. We just need to cling to faith in Jesus as our Hope and certainty - and get our focus off our how we FEEL concerning our own shortcomings. These are only what the DEVIL wants us to believe our salvation is dependent on - not in a faithful, loving God Who died for us! God wants us to have CONFIDENCE we are His, and in our salvation, and in HIS faithfulness. Does He save one whom desire to be saved and follow Him, only to abandon us back to our own pathetic ways? NEVER!