This is the spot where the little baby born in Rome, Eugenio Pacelli, received his Holy Baptism as a newborn. The church here is only open Sunday mornings for Holy Mass, but be sure to visit this site and pray here in the Church of Ss. Celso e Giuliano on the Via del Banco di S. Spirito, 6.
N.B. For reasons unknown to the rest of us, at some point (likely in the seventies?) they took the original font out of this church and moved it to the Basilica of San Pancrazio (see photo below).