John the baptiser knew that though his ministry was effective, it was limited by definition. The baptism of water could cleanse people from their past sins but could not keep them clean or offer them a motivation against future sin. His baptism had to be supplemented with a power that would produce purity and unity, the power to be and to do. 

As a result, John preached expectantly about the baptism in Holy Spirit, which will complete the first by supplying the power to rule over sin and to overcome the world.