Peter's sermon to such a large crowd of Jews reminds us of stories of how God's Spirit used George Whitefield in the 1700's as an instrument to bring about the First Great Awakening in America.
John MacArthur says that "Whitfield preached in Bristol to 20,000 people regularly, often daily. Sometimes when Whitfield got up in the morning, there would be 10,000 people outside waiting for him to begin. Often, he preached to 40,000 people.
It is said that near Glasgow, Scotland he preached to as many of 50,000 to 100,000 people. Now this was done without any publicity (AND WITHOUT A PA SYSTEM!). This is just people drawn to the power of the Word of God."