Now while he was in Jerusalem at the Passover Feast, many people saw the miraculous signs he was doing and believed in his name.
The people believed because they saw the signs Jesus did. This is only a first attraction to the Lord and not a deep faith, but it is certainly a wise decision on the part of the believers to put their trust in Him because of what He did. They put their trust in him because He was doing great things nobody else could do and these things all pointed to God.