Modalists held that God was initially the Creator, then became the Redeemer, then became the Spirit at Pentecost. The divine person who came to earth as the incarnate Jesus was the same person who had created all things. When He returned to heaven, He took up His role as the Father again, but then returned to earth as the Holy Spirit. As you can see, the idea here was that there is only one God, but that He acts in different modes, or different expressions, from time to time.