Jesus said they would be baptized with the Holy Spirit. To be baptized is to be immersed in something. They had yet to be immersed in the Holy Spirit. That immersion happened 50 days later as they waited in the upper room. It was a dramatic event that changed their lives forever. They were transformed from insecure to confident, from weak to powerful.
Many Christians today have not been immersed in the Holy Spirit. How about you? Have you experienced Him? When you do, you will know it!