They do so to Christ personally present in the Holy Eucharist. When Christ allowed St. Thomas the Apostle to touch the wounds in His hands and feet, St. Thomas said, “My Lord and my God.” The same Christ left Himself present in the Eucharist when He said, “This is My Body,” and when Catholics come into His presence they offer Him the tribute of their deep reverence and worship by genuflection. You, too, would kneel before Christ, if you believed as Catholics do.
Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty