They were sure of who they were because they were sure of Christ. They were thoroughly convinced, soundly converted, and Holy Spirit-filled believers. Imagine for a moment, sitting in a packed Raymond James Stadium in Tampa, just as your home team wins. Imagine the excitement in the air as the home team scores the winning touch down in overtime. But imagine for a moment, being the only one seated in that empty stadium when that winning touch down is scored.
Excitement breeds excitement.
However, the euphoric excitement of your favorite football team winning would pale into significance when compared to the excitement on Pentecost. There is no comparison of me winning vicariously through the men on the field and what I have won through the vicarious death of God's Son. Now there is a brand new life, which can be had each day of my life. When Christ won that victory over Satan on the cross, it was my personal salvation that was won. He won for me a new life filled with glories of God's grace.