May have once been a hall of the Sessorian Palace, Saint Helen, the
mother of Emperor Constantine, built or adapted it to a church in 320
to receive the large relic of the cross that she had brought from Jerusalem.

Several well-known relics are housed in the Cappella delle Reliquie, built
in 1930 by architect Florestano di Fausto, including: a part of the panel
which was hung on Christ's Cross; two thorns of the crown; an incomplete nail;
and three small wooden pieces of the True Cross itself.