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. (John 14:12)
After His departure He would send the Holy Spirit (v. 16; ch. 16:7), who would endue the disciples with power (Luke 24:49). As a result of the Pentecostal outpouring, and subsequent ones, the gospel was heralded with great power, so that about 40 years later Paul could say that the gospel was “preached to every creature which is under heaven” (Col. 1:23; cf. DA 633).
Greater works.
That is, greater in quantity rather than quality. Christ’s activity had extended over a relatively small area (of the world). After the ascension the gospel would spread to all parts of the world.