All cults and man-made religions are based in “works righteousness.” Their adherents believe they have to do something to earn their way to heaven — pray five times a day, lie on beds of nails, do good works, fast, repeat certain prayers, etc. They do this because they are ignorant of God’s standard of righteousness. This is why they need the Law of God to show them that the leap they are trying to make is infinitely wider than the Grand Canyon. However, before you take them through the Law, help them see that they are indeed trusting in “works” for salvation.
Let’s say there are a couple of cult members at my door. I warmly ask for their names, and then say, “I have a knife in my back. I am dying and have only three minutes to live. What do I need to do to enter heaven/paradise/the kingdom of God?” They look concerned. One says, “A lot.” I ask, “What do you mean ‘a lot’? I have only two minutes to live. Help me.” They will normally say they cannot help someone who has just a couple of minutes to live, because their salvation is based on gaining knowledge and doing “good works.” The fact that they must do things to be saved reveals that they are trusting in their “self-righteousness.”
When I then ask if they think they are “good” people, they almost always say they are, and that is the root cause of their deception. While they know they are sinners, they believe their sin is not so bad that they cannot earn their own way out of it and “merit” heaven. So they must be taken through the Law and made to understand that they are criminals in the sight of a holy Judge, and are guilty of countless crimes. They must be taken through the Law and made to understand that they are criminals in the sight of a holy Judge, and are guilty of countless crimes. They must understand that God is perfect and holy, that He considers lust to be adultery and hatred to be murder, and He will see to it that absolute justice is done. That means adulterers, murderers, liars, and thieves will be damned forever. Once they recognize that, they will understand that their “good” works are not good at all, but are in reality a detestable attempt to bribe the Judge of the universe. Hopefully they will trust in His mercy alone to save them.
That is how the thief on the cross was saved — through mercy alone. He didn’t go anywhere or do anything to save himself. He couldn’t, because he was nailed to the cross. He had no other avenue but to humbly turn to Jesus and say, “Lord, remember me when You come into Your kingdom” (Luke 23:42). In doing so, he acknowledged Jesus as Lord, and believed that He would rise from the dead (Rom. 10:9).
That is all that any who are involved in “works righteousness” religions need to do to be saved. They are condemned by the Law. They cannot go anywhere or do anything. All they can do is turn to Jesus and trust in Him alone for their salvation. We are saved by grace through faith, and that not of ourselves; “it is the gift of God, not of works, lest anyone should boast” (Eph. 2:8,9). So plant that seed in the hearts of those who think they can be saved by their own works, then pray that God causes it to grow and produce fruit.