“But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words:” — Acts 2:14
The ability of Peter to stand up and boldly expound scripture is amazing in this chapter. He will continue to be used of the Lord throughout the days of the early church, taking the lead, preaching the gospel, doing miracles, and giving direction for the church.
His ability is especially remarkable when you consider how many mistakes and missteps He had during the ministry of Jesus. His outright denial of Jesus only a few weeks earlier greatly contrasts with the boldness we now see.
No doubt two factors make all the difference: 1) the resurrection, and 2) the filling of the Holy Spirit. The resurrection filled him with conviction and the filling of the Holy Spirit filled him with power and wisdom to preach the gospel.
Just as God took a hard-headed, uneducated, and, at times, confused fisherman from Galilee and turned him into a bold preacher of the gospel, so God can fill us and equip us for ministry. Belief in His resurrection and power from His Holy Spirit are the fuel that will propel us forward into the world.