The following is an excerpt from
The MacArthur New Testament Commentary on John 14.
Truly, truly, I say to you, he who believes in Me, the works that I do, he will do also; and greater works than these he will do; because I go to the Father. (John 14:12)
The astonishing promise to the one who believes in Christ is that the works that He does, he will do also; and greater works than these he will do. The greater works to which Jesus referred were not greater in power than those He performed, but greater in extent. The disciples would indeed perform miraculous works, as Jesus had (cf. Acts 5:12–16; Heb. 2:3–4). But those physical miracles were not primarily what Jesus had in mind, since the apostles did not do more powerful miracles than He had. When the Lord spoke of His followers performing greater works, He was referring to the extent of the spiritual miracle of salvation. Jesus never preached outside of Palestine, yet His followers would spread the gospel throughout the world. Jesus had only a limited outreach to Gentiles (cf. Mark 7:26ff.), but the disciples (particularly Peter and later Paul) would reach the Gentile world with the gospel. The number of believers in Christ would also grow far beyond the hundreds (Acts 1:15;1 Cor.15:6) that were numbered during His lifetime.
The power to perform those greater works would only be available because Jesus was going to the Father. It was only then that He would send the Holy Spirit (John 7:39; cf. 14:16–17, 26; 15:26; 16:13; Acts 1:5) to indwell believers (Rom. 8:9–11) and empower them for ministry (Acts 1:8;1 Cor.12:4–11;cf.Eph.3:20).
Christ’s promise to send the Holy Spirit offered further comfort to the disciples. Though Jesus would no longer be visibly present with them, the Spirit would provide them with all the power they needed to extend the work He had begun (cf. Acts 1:8).