Why should you keep in memory the Gospel you were taught?
Keeping the Gospel in memory matters because it is the message by which believers are saved and stand. It anchors them against every wind of doctrine and false teacher, and guarding it prevents a vain, empty belief. The Gospel is their salvation, so remembering it secures their stand and eternal hope.
Les teaches from I Corinthians 15:1-2 that the Gospel preached, received, and stood upon is the same message by which believers are saved. He explains that standing on this Gospel keeps a person from being blown over by every little wind of doctrine and every false teacher, so they do not have to worry about eternity. Les also stresses that the word "saved" is not a denominational word but a Scriptural word, reaching back to the Old Testament expectation of salvation. Using the analogy of a man saved from bankruptcy by a rich uncle's estate, Les says salvation means being saved from something, and this Gospel saves from eternal doom. Therefore, if you have kept in memory what was preached, it is lest you believe in vain, so know what you believe.
By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.