When you are given the opportunity to sit down and share with a cult member, begin your conversation with prayer. This will not only be an obvious testimony to them that you are serious about your life with Christ, but will give them cause to think. As you pray aloud, direct your prayer towards man's own lostness and God's infinite love as He shed His blood for our sin through Jesus. Ask the Lord that His Spirit would reveal truth to both parties in the discussion. The best skill in argumentation is no match for the anointing of the Holy Spirit, and your prayer will set the stage for the course of your discussion.
Many men are sincere, but they can also be sincerely wrong.
Many times cultists who seem impervious to seeing doctrinal or theological errors will come out of a cult through being shown dishonesty, deception, or unethical behavior by the leader. Keep your cool. Then you can discuss it later and you will be on firmer ground after more study. As with any other sinner, the last thing he wants to do is submit to God and receive Christ as Savior. You must make the demands of the gospel clear, and zero in on that issue.