"About 40 years ago I had an experience where I wrote a tract"

Tim LaHaye is describing an experience he had about 40 years ago when he wrote a tract. A system is a short religious or political pamphlet, typically consisting of an appeal to the reader to take some action. In this case, it is most likely that the system was written to spread out a spiritual message. It is possible that the tract was written to spread the gospel of Jesus Christ, as LaHaye is a Christian minister and author. He may have composed the system to share his faith with others and to motivate them to accept Jesus as their Lord and Savior. It is also possible that the tract was written to spread out a political message, as LaHaye was also an active political figure. Whatever the message of the tract was, it is clear that it was an essential experience for LaHaye which it had an enduring impact on him.