A Functional Definition Of Sunday School: Sunday School is an essential disciple-making strategy of the Church. As such, Sunday School has proven its effectiveness by teaching biblical principles and godly living to people at every age and stage of life. In order for Sunday School to remain effective in the 21st century, the Church must both reaffirm and retain the essential elements of this disciple-making strategy while also developing flexibility to address the diverse discipline challenges of our fast-paced, pluralistic culture.