The original dedication of St Mary's Church, Cusop was to the Welsh saint, St Cewydd, the Welsh Rain Saint. In fact the name Cusop is derived from Cewydd. Cewydd was the son of Caw of Prydyn (Pict Land) whose family, having been expelled from their territory in North Britain, sought refuge in Wales. Cewydd remains the patron of churches in Aberedw and Disserth.
Features of the original Norman Church can still be seen. There is Norman carving on the chancel arch and the small window to the west side of the main door is Norman. The north door is blocked but can easily be seen from outside, at the back of the church. There was once a rood loft and screen, in front of the chancel arch, but all that remains is the built-in doorway above the war memorial.
The font may date as far back as the 14th century. It is made of a single massive stone decorated with saltire crosses.
The roof of the church was constructed in the 14th century. In 1961 a new bellcote was erected with six new bells. In the year 2000 a new Millennium window was designed by Nicola Hopwood and installed in the east window.
The presence of the magnificent, ancient yew trees in the churchyard raises the probability that the church was built on a former Pagan site; yew trees were considered to be symbols of fertility and immortality in Pagan times.