User blog comment:B214/Volume 23 discussion blog/@comment-77.188.244.180-20170422171209/@comment-30924344-20170422172245

A sword is only strong by how strong its wielder is. Issei using the power of Ascalon would be deadly against Crom Crauch. He did not give it back because its his and Michael gave it to Issei not only as a gift to show he was in agreement with the peace treaty, but because the angels have no need for it. It is the sword of a Saint and saints are very important to christianity so Issei having the sword means they could give it away.