They had been redeemed. They were willing to follow Moses (though many times, they wished they hadn't). They had some sincerity in originally having chosen to follow Moses. It is the same way, when people choose to follow God. As can be seen in the demonstrations of the parable of the Sower, and the various soil types. A person can easily be sincere in choices they make in life. They may feel overwhelmingly certain that what they have found is good and something they want. So in that way, at that point, can be called, "sincere" - in this feeling and decion they've made. But some things that we begin with the conclusion that something we've found is right and good - we later come to decide that we don't want after all. Examples: Seeking a mate - sometimes, later on in the relationship, one comes to see that the one they're dating is not the right one to marry, or not as desirable as they'd at first assumed - so he or she breaks off the engagement. It is proper to discontinue something we later discover is not something that would be good for us. Another example: Sometimes, we buy something we overwhelmingly think is desirable and "just what we need" - but later on, we regret we had bought it, and get rid of it.
The same thing often happens in that matter of choosing to accept God's gift of salvation. As can be seen in the parable of the Sower. Some of them who begin with Christ, faithfully continue on throughout hardships & temptations they encounter - without falling away from Christ. But others of them, turn away from Christ, after having been tried by hardships or temptations long enough for it to be possible to determine how faithful they will eventually will be - after that initial starting period.
No one helplessly falls away from Christ, as we all have the ability to make choices in life - God has made it that way. Everyone who chooses to remain faithful to Christ, has ways to help keep himself faithful in Christ - ways the Bible tells us of: Regular prayer, putting on the whole armor of God regularly, etc. God has explained all of that to us, so that we have plenty of ways to keep ourselves faithful to God to the end of our lives.
Paul had full certainty (Philippians 1:23) that he would end up in heaven and that he wouldn't fall away from Christ in this life. That's because he chose to make pleasing God in this life, the most important thing he wanted. He had good certainty, but not complete, absolute certainty about Timothy - it seems, since he also warned him to be careful to remain true to God (I Timothy 6:15). But that shouldn't be surprising - none of us knows what's in other people's hearts as well as God can. All we can know with certainty, if we prayerfully and sincerely serve God and have been tried enough - is that we unbendingly plan to continue to be faithful to Him throughout the end of our lives, and that therefore, God has given us the power to do so.
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