Hebrews 3:15-19 (ESV)
As it is said,
"Today, if you hear his voice,
do not harden your hearts as in the rebellion."
 For who were those who heard and yet rebelled? Was it not all those who left Egypt led by Moses?  And with whom was he provoked for forty years? Was it not with those who sinned, whose bodies fell in the wilderness?  And to whom did he swear that they would not enter his rest, but to those who were disobedient?  So we see that they were unable to enter because of unbelief.
There is a strong warning here. The men who rebelled were precisely the ones rescued from slavery in Egypt. They saw the great miracles that God performed through Moses. Yet, they rebelled. They lost faith. It is possible. You can lose your faith. If you couldn’t there would be no need for this warning. Unbelief, that is the danger. One can be led from faith to unbelief through sins. Either outright immorality, or hedonism. Or through neglect of faith. Which is why pastors worry about the members they don’t see in church much. Faith is a precious gift from god, it should be cherished nod nourished. God does this through the Church.