The cornerstones are characteristic Saxon 'long and short work', using both tall stones with occasional flat ones to hold the corner together. On the east and west sides the bell-openings are topped not by arches but by pairs of stones leaning together to form a triangle, another typically Saxon feature. The church is also notable for some crude but vigorous carvings.
William de Braose, lord of Bramber, gave the church, its lands and its tithes to the Templars in 1154, making it part of a Europe-wide operation devoted to supplying men and money for the fight to keep Jerusalem in Christian hands. In about 1180, Sompting church was adapted to its new role by the addition of a private chapel on the south side, and a transept on the north side.
The south chapel was a large square room with a shallow, vaulted sanctuary at the east end. Originally, it was completely separate from the church, accessed only through a small door leading from the sanctuary into the chancel.
The north transept has two lovely arches on the east wall to form a pair of chapels, also for the use of the Templars. The main nave was used by the parish - part of the condition of the gift was the Order had to employ a vicar for the local people.
The church would have been used as a centre for administering the Order's land, collecting tithes, recruiting fighting men and as a place for travelling members to stay. In many ways it would have been a local branch of what has been described as the world's first diversified multinational corporation, with subsidiaries in farming, industry and banking.When the Templars were suppressed in 1324, Sompting was passed to the rival Hospitallers and finally became an ordinary parish church at the Dissolution of the Monasteries in 1540. In Victorian times, the wall between the Templar's chapel and the nave was punched through by a large arch, so it now looks more like a south transept.