Some understand Acts 8 to be saying that these people in Samaria were Christians, and that they needed to be given that special, unique-for-this-time experience of the Holy Spirit—it may well have been that. Just because we say that true conversion, by definition, must involve the Holy Spirit’s regenerating work, does not mean that we do not pursue experiences of the Holy Spirit in the Christian life again and again and again.
We must plead for the Holy Spirit to come down—whether you want to call that a prayer for many baptisms of the Holy Spirit throughout the Christian life, or simply filling of the Holy Spirit (which is an easy-to-understand term for it). Whatever you want to call it—revival is another helpful word.
We must pray that the Holy Spirit would come down and change us who are already Christians—and that he would also change those who are not yet Christians for the first time.