A testimony of a changed life is a powerful thing, and every believer has one. Your testimony is the story of how Jesus Christ has saved you and changed you and made you new. The details of my testimony will be different than the details of your testimony, but there is a common thread that runs through every single one. That common thread is this—I was once lost in my sins, but Jesus came and found me and rescued me by His grace.
The gospel is the good news of what God has done in Christ to reconcile broken and helpless sinners to Himself. That is what happened in Acts. The second chapter closes with an amazing picture of the church, one in which the Lord is daily adding to their number those who were being saved. It closes by mentioning the many who were being saved, but chapter 3 begins by focusing on the one. Those who reach the many care about reaching the one.