Acts 1: They pray in the Upper Room for 10 straight days; the Holy Spirit comes, Peter preaches, and 3000 are saved.
Acts 4-9: They pray, and God fills them with such boldness that they turn the city of Jerusalem upside down. The church in Jerusalem grows to over 10,000 people and some of the harshest critics (like the Jewish priests, and eventually Paul himself), are getting saved.
Acts 12: They pray, and God orchestrates a prison break.
Acts 13: They pray, and God raises up Paul to be a missionary, the greatest missionary the world has ever known.