This is the Temple of Antonio and Faustina. The door you can see through the columns used to be at ground level in the middle ages, when this building was converted into the church of San Lorenzo in Miranda.

The Emperor Antonius Pius built this temple for his wife after her death in 141 A.D. because he desperately missed her and wanted to deify her.

After the Emperor's death, the Roman people used it to honor him as well. Apparently the didn't agree that his wife was the only deity in the family -- in fact, in true Roman fashion they put his name ahead of hers despite his wishes.