JOEL - John 14:12 says:

"Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father"

Certainly there was no greater works than the atonement, the resurection, the establishment of His gospel on the earth, and the creation of the universe. I believe Jesus meant that the works believers will do on the earth will be greater in terms of quantity rather than quality. Jesus said that they will do greater works "because I go unto my Father". His was a relatively short life on the earth, and a short ministry(3 years). He knew that after He died others would come along who would labor a much longer time on earth and produce many more fruits than He did. For example, Paul the apostle probably converted more people than He did. The church today has spread the gospel over a larger part of the earth than both He and Paul did during their earthly ministries.