The presence of a centre for Christian worship in the old parish of Ruabon has been hallowed by its association with the Celtic saints. Edward Lhuyd, writing in about 1697, says 'They call a field, where there is a cross in Ruabon parish, Kappel Kollen and a field Cae'r groes newydd and another Errow armon'. Nearby is the field cae gosper, meaning 'Evensong or Vespers'. There may have been a place of worship in this western township of Dininlle Issa of which these field names recall the Saints Collen and Garmon. The place name Rhiwabon, the hillside or slope of Mabon, refers possibly to the sixth-century St Mabon the Confessor, son of Tegonwy ab Teon and brother to St Llewelyn of Welshpool. The name Mabon means a 'boy' or 'youth'; in old Welsh it is Maponos, which as Apollo Maponos occurs as the name of the Celtic Sun god.
The site of the existing church in the township of Rhuddallt is surrounded by the neat circular churchyard which formerly extended to the west down Ysgoldy Hill and is a rare survival in Wrexham Maelor of a llan -'the church within an enclosed cemetery', 'the township surrounding a parish church'. The township of Rhuddallt lies between Offa's and Wat's dykes, the boundary between the Welsh and English from the eighth century. The site of the church is therefore pre-Norman, possibly a seventh-century foundation, dedicated to St Collen. In the Valuation of Norwich (1254) it is described as the Church of St Collen, Ruabon. By the end of the thirteenth century the parish had been appropriated to the Cistercian Abbey of Valle Crucis and the church re-dedicated to the Blessed Virgin Mary with the Feast of the Assumption (15 August) as its festival.