As described in Matthew 14:28-29, Christ sent his disciples ahead in their
boat whilst he retired to pray. When they later saw him walking across
the stormy waves to join them they feared he might be some spirit, prompting
Peter to call out "(28) ...Lord, if it be thou, bid me come unto
thee on the water. (29) And He said, Come. And when Peter was come down
out of the ship, he walked on the water, to go to Jesus." The artist
here has shown Peter in the process of stepping out of the boat.