When the people heard this, they were cut to the heart and said to Peter and the other apostles, "Brothers, what shall we do?" Peter replied, "Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. The promise is for you and your children and for all who are far off-- for all whom the Lord our God will call." With many other words he warned them; and he pleaded with them, "Save yourselves from this corrupt generation." Those who accepted his message were baptized, and about three thousand were added to their number that day. Acts 2:37-41
So it seemed that despite the fact that Peter proposed that these people had killed Jesus just over a month earlier and they knew that he did miracles, 3000 believed. But what if what Peter said was false? What if they knew that Jesus hadn't done miracles? Why would they have believed? They wouldn't have!