This is the central event of Pentecost: the coming of the Holy Spirit in power. Before he ascended to heaven, Jesus breathed on the disciples and gave them the gift of the Spirit (John 20:22) but they were not filled with the power of the Spirit until the moment of Pentecost. Before this, throughout the Old Covenant, the Holy Spirit descended on men like Elijah, Elisha, Moses and even Saul (1 Sa 19:24), but it was only temporary.
Once his work was done in a person’s life – whether for a moment or years – the Holy Spirit departed from that person and returned to the Lord. At Pentecost though, the Holy Spirit descended and stayed. He is actively in the world in the hearts of every believer today and will remain so until Christ returns at the rapture and the Holy Spirit once again returns to heaven with the Church (2 Thess 2:6). Only at that moment will his ministry on Earth be completed.