Matthew Henry commentary on John 14:12:
“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.”
That they should do greater works than these.
[1.] In the kingdom of nature they should work greater miracles. No miracle is little, but some to our apprehension seem greater than others. Christ had healed with the hem of his garment, but Peter with his shadow (Acts 5:15), Paul by the handkerchief that had touched him, Acts 19:12. Christ wrought miracles for two or three years in one country, but his followers wrought miracles in his name for many ages in divers countries. You shall do greater works, if there be occasion, for the glory of God. The prayer of faith, if at any time it had been necessary, would have removed mountains.
[2.] IN THE KINGDOM OF GRACE. THEY SHOULD OBTAIN GREATER VICTORIES BY THE GOSPEL than had been obtained while Christ was upon earth. The truth is, the captivating of so great a part of the world to Christ, under such outward disadvantages, was the miracle of all. I think this refers especially to the gift of tongues; this was the immediate effect of the pouring out of the Spirit, which was a constant miracle upon the mind, in which words are framed, and which was made to serve so glorious an intention as that of spreading the gospel to all nations in their own language. This was a greater sign to them that believed not (1 Co. 14:22), and more powerful for their conviction, than any other miracle whatever.
~ A.W TOZER