The construction of the Holy Trinity Basilica in Chełmża, which is currently one of the oldest brick structures in the Chełmno Land, was started on July 22, 1251. Its initiator was the bishop of Chełmno, Heidenryk, who issued a decree on the creation of the cathedral of the Chełmno diocese. In the second half of the thirteenth century, the construction of the temple was interrupted by frequent attacks of pagan Prussian tribes. In 1286 additional destruction caused a fire. After the damages was repaired, further construction was started, but the original design was abandoned. The completion of the main structure of the church is dated to 1359, and the whole at the end of the 14th century.
The cathedral suffered greatly during the invasion of Polish-Lithuanian troops in 1422. Its reconstruction was prolonged so long that even the proposal to move the episcopal capital to Chełmno was made. However, this project collapsed, and the reconstruction of the cathedral was finally completed in 1692. In 1821, the cathedral in Chełmża was demoted to the rank of a parish church, and in 1982 it was raised to the rank of a smaller basilica. In 1906, the original gothic western portal was replaced with a neo-gothic porch. In 1950 a fire broke out which destroyed the roofs, part of the vaults of the nave, the tower’s helmet and the chancel arch. Reconstruction in the years 1968-1971 restored the church to its former appearance.
The gothic church of the Holy Trinity is a orientated brick and hall building. In a horizontal view it consists of a rectangular chancel with a baroque sacristy added from the south, a slightly separated transept together with two turrets adjacent to its arms and a three-nave corpus ended on the western side with a two-tower façade. The entire temple is dominated by a northern tower with a height of 28,5 meters, named after the initiator, “Opaliński Tower”.
The interior of the church is covered with stellar vaults supported in the chancel on the perimeter walls, and in the main corpus on the freestanding, inter-nave pillars and side walls. The oldest among the existing ones are the stellar vaults of the two-span chancel. The Chełmża stellar vault in the presbytery can be considered as one of the oldest in Europe.
Medieval equipment of the temple is a fragment of the tombstone of the Grand Master of the Teutonic Order Siegfried von Feuchtwangen, group of Crucifixions from 1422 and sculptures of the former chancel arch from the 16th century.