The original dedication of the church was to St Merryn but in Norman times the saint was assumed to be a St Marina ("see above", Toponymy). When the sainthood of Thomas Becket (Thomas of Canterbury) was ratified by Rome in 1338, a second dedication was made to him but it did not replace the St Merryn dedication. The first resident Vicar, John de Withiel, was installed on 2 July 1259. The church building is of Norman foundation but the chancel, south aisle and upper part of the tower are of the 15th century. The font of Cataclewse stone, quarried on Trevose Head, has carved figures of the twelve Apostles; It originally belonged to the chapel of St Constantine in the parish. Charles Henderson dated it as c. 1420 and gives the date of rebuilding the chancel as 1422.