We remember that the disciples received the Holy Spirit when Jesus breathed upon them. Then we know that they were sealed and had the Holy Spirit in them. But next, Jesus said, "Behold, I send the promise of my father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high (Luke 24:49)" for "ye shall be baptized with the Holy Ghost not many days hence (Acts 1:5)." How could they be baptized after they had been baptized into His body when they received the Spirit?
The answer concerns the Greek word bapto, which was a common term used in the garment industry. It has the connotation of being made fully wet with fluid in the manner of staining as with a dye. The disciples had received the promise of the Spirit in their hearts (2 Corinthians 1:22) and now they were to receive the promise in the form of power "upon" them. This is shown by the fact that "all were baptized unto Moses in the cloud (1 Corinthians 10:2)." "A cloud covered the mount (Exodus 24:15)." "And Moses went into the midst of the cloud, and gat him up into the mount (Exodus 24:18)."
Therefore, the word baptism applied here is a picture of another "immersion" of the believer into an additional truth or capacity within God. The Holy Spirit comes "upon" him as if the Spirit has approached (ie. come upon) him in a new way. We remember that the believer "entereth in (Ezekiel 46:9)" to a new worship experience or meeting with God in each of the three feasts of the Lord (Exodus 23:14).