Nothing is known of the first Church of St Erth, or of others which may have succeeded it until the present Church was originally built in the 14th or 15th centuries. The present Nave has some early Perpendicular work, with the Chancel a little earlier in the Transitional period. The Tower, which is without buttresses, is 14th century. There are six bells, the tenor bell having been re-cast by Harvey & Co Ltd, Hayle, in January, 1901.
The difficulty of dating the Church with accuracy arises from the extensive restoration and re-building carried out. Vicar Collins, in 1747, showing great zeal in his removals and repairs, and even more extensive re-building was instigated by Vicar Mills in 1873. How great this was can be judged by a Press report of the re-opening of the Church in 1874 stating: "The work taken in hand was so extensive a character that of the old building all that now remained are the tower and pillars, the latter having had to be extensively restored.
Altar, reredos and roof are all brightly decorated with painted carvings of many interesting subjects. The reredos, unusually, has the Adoration of the Magi as its central subject flanked by four Cornish Saints: Petroc, Erth, Piran and Conan. There was a tapestry incorporated in the wall decoration which has been returned to Trewinnard Manor from whence it came.
The walls are built of polyphant stone, the old high pews, which were described as modern cattle pens, are succeeded by open benches of varnished pitch pine, the chancel roof has been beautifully decorated, and the chancel paved with encaustic tiles.
The old windows have been worked in with the exception of that of the East end, which is entirely new." It may well be this account is a little exaggerated. It is recorded elsewhere that it had been decided to rebuild the east and south walls, and the other walls may have been less drastically dealt with.
The Chancel was extended a few feet eastward, and as well as the new window already mentioned, a new doorway was built in the porch using the same dark stone. The floor level was raised considerably, and the bases of the pillars are now mostly buried. Much of the old timbering can be seen in the roofs of the aisles,, some of it still with traces of the bright colours which covered the original beams.
It is unusual to have dormer windows in a church. St Erth has two, which were inserted early in the 20th century, each with a pair of angels carved at its corners.