What does John 14:20 mean?
In this passage, Jesus has promised the coming of the Holy Spirit (John 14:16–17). That reference implied that this Spirit of Truth "would be" in believers, presumably looking forward to Pentecost (Acts 2:1–4). This event will occur after Jesus' resurrection, and will mark the beginning of the Holy Spirit's indwelling ministry.The reference to "that day" here has been interpreted in various ways. Some connect this to Jesus' prior comments about the Holy Spirit, making it a reference to Pentecost. Supporting this view is the fact that Jesus continues to speak of being "in" believers, and Christ being "in" God. Another strong possibility is that "that day" refers to the moment when the disciples "will see [Christ]" after His resurrection (John 14:19), as mentioned in the last verse.
Taken along with verse 16, this statement also continues to provide support for the doctrine of the Trinity. Jesus promised that the Helper "will be in you" (John 14:17), while here Jesus says "I [will be] in you." This is the consistent pattern of the Bible, where the Father, Son, and Holy Spirit are spoken of with the same attributes.