That was the standard greeting of the early church. Whenever two Christians would meet they would remember the glorious fact that Jesus rose from the dead, that He proved that He was who he said He was, that He had authority in heaven and on Earth, that He had authority over death, hell and the grave.
Whenever they would meet, one would greet the other with "He is risen!" and the other would respond, "He is risen indeed!"
What a glorious greeting!