This is the oldest church in Jerusalem, built in the mid-5th century and restored after the Persians destroyed it in 614. It is accessed from Christian Quarter Rd, near where it intersects with David St. The church isn't usually open to visitors, but monks in the adjoining Greek Orthodox complex (of which the church is now a part of) have a key; alternatively, with advance notice, the tourist office can assist with access.
In the 11th century, the merchants of Amalfi used the walls of the original building when constructing a new church, which became the cradle of the Knights Hospitaller; wounded knights were cared for within. It was briefly repurposed as a mosque in the 16th century.
It's not easy to spot; look for the small relief of the head of John the Baptist above the doorway. You have to duck your head to enter.
