The purpose for classes is focused and tailored instruction in the ways of the Lord. Particular attention should be given to the foundations of the faith. Our people are not grounded, and there is no acceptable excuse for this condition. Almost inadvertently, we have produced a body of people that lack spiritual roots. One of the results is the prolific increase of counselors and "how-to" seminars. There is an increased requirement for these things because people are not able to cope with life. Our failure to ground them in the faith has made them vulnerable to the artifices of the devil.
Believers will not be able to "stand against the wiles of the devil" (Eph. 6:10ff) if they are not established in the faith. And, if they cannot stand, they will fall! It makes little difference what may appear to have been accomplished in people if they cannot stand in the "day of adversity!" Solomon wisely said, "If thou faint in the day of adversity, thy strength is small" (Proverbs 24:10). Jeremiah provides a telling observation in this regard: "If thou hast run with the footmen, and they have wearied thee, then how canst thou contend with horses? and if in the land of peace, wherein thou trustedst, they wearied thee, then how wilt thou do in the swelling of Jordan?" (Jeremiah 12:5). The idea is this: if the ordinary difficulties of life disorient you, what will happen when severe trials come to you? The remarkable rise of immorality and erratic behavior in the church tells us that people are not able to contend with the unusual.
Further, believers are to "grow in grace, and in the knowledge of our Lord Jesus Christ" (2 Pet. 3:18). No alternative to spiritual growth is provided by our Lord--not if you expect to survive the attacks of the evil one and master your own weaknesses. Growth is necessary for survival. You simply cannot remain in spiritual infancy and stand under the assaults of the "prince of the power of the air" (Eph. 2:1-2_! Our Lord declared that those hearing and receiving the Word, yet lacking root in themselves, were unable to endure the heat of the day. His telling remark is this: "And have no root in themselves, and so endure but for a time: afterward, when affliction or persecution ariseth for the word's sake, immediately they are offended" (Mark 4:17).
All of this does have a bearing on the Sunday School. Here is where instruction in the manner and requirements of the kingdom takes place in a unique way. Your classes should be tailored for your constituency. If you are a young congregation, with most of your people being new in Christ, your curriculum should be structured to ground the people. It is not enough to simply roam through the Bible, dealing with generalities. Target making your people strong believers that can stand against the wiles of the devil.