From what I understand of their belief is that once one accepts Jesus Christ into their hearts and pray the ‘sinners’ prayer then there is no sin, no evil this individual can do so as to keep them from reaching heaven. They only need to believe in Jesus (John 3:16) and they’ll be ‘saved’. They have absolute assurance of salvation. And since works do not gain our salvation, our works cannot contribute to our losing our salvation. We are now “hid” with Christ in God (Colossians 3:3). There is “no condemnation” for those in Christ (Romans 8:1-2).
In fact some also bring up John 10:27 as proof that they can never lose their salvation since Jesus proclaims that “My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.” And yet nowhere do we find that one cannot willingly jump out of His hands through his own choosing.
Re-read 2 Peter 2:20-21. This letter was written to the believing Jews throughout the known world. Here is what he says to those who lose there way:
"If they have escaped the corruption of the world by knowing our
Lord and Savior Jesus Christ and are again entangled in it and
overcome, they are worse off at the end than they were at the
beginning. It would have been better for them not to have known
the way of righteousness, than to have known it and then to turn
their backs on the sacred command that was passed on to them."
These people who escaped the corruption of the world were saved because they escaped the corruption by knowing our Lord and Savior Jesus Christ! These saved people who became entangled AGAIN in it and overcome...these people are worse off than they were in the beginning because they knew the way of righteousness and they turned their backs on the sacred command passed on to them.
Peter can only be talking about certain saved Jews who turned away from righteousness and now they are worse off than they were before knowing that way of righteousness. They lost their salvation even after finding it.
Jesus also speaks of this loss and promises their acceptance if anyone repents and comes back after rejecting the father. The Parable of the lost son speaks of a son who rejects his father and asks for his inheritance in advance essentially telling his father that he is dead to him. The father in the parable is the Lord and He gives us the free will to choose or reject Him. But even when one rejects Him, our Father in heaven is always waiting for our repentance and return because when we turn away we are dead and when we return to Him we are “alive again” (Luke 15:31).