Jesús is the one the Jesuits never finished. They were still building the church when the expulsion order arrived from Madrid in 1767, and the masons simply put down their tools. The walls stand to full height, the great trefoil arches are cut and set, and there is no roof and never was one.
The church was designed as a copy of the sanctuary at Loyola in Spain, 70 m long and 24 m wide, which would have made it one of the largest in the missions. The three Moorish-looking trefoil doorways on the north front are unlike anything else in the Paraná missions and are the reason photographers come. The corner tower stops abruptly at the height the builders had reached.
The site is quieter and emptier than Trinidad, 12 km away, and it feels different: Trinidad is a ruined town, Jesús is an interrupted building site. Most people find it the more affecting of the two, partly because there is usually nobody else there.
Your Trinidad ticket covers it. Buses are infrequent on the Jesús branch road; if you are not driving, most people take a moto-taxi or a shared car from the Trinidad crossroads, or come on a tour from Encarnación that does both. Allow an hour, more if the light is good.