“What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?” (1Co 6:19 KJV)
Jesus had to ascend to Heaven in order to send the Holy Spirit. While He was in this world, the spirit of God was in Him. When He returns to Heaven, He can then send His spirit to live in each of us who believe in Him.
“Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.” (Jhn 16:7 KJV)