Theologically speaking, Pope Sixtus III sought to heal divisions between orthodox and heretical believers, especially in the wake of the Council of Ephesus. Sixtus himself, before he became pope, had been sympathetic to the teachings of Pelagius and only with some effort was able to adopt orthodox beliefs on the nature of salvation. This likely encouraged him to try to be more sympathetic to heretics generally - seeing them as honestly misguided rather than deliberately promoting error. He seems to have made great strides when it came to the Alexandrians, the Netorians, and the Antiochenes.
Politically, Pope Sixtus III was heavily involved with repairing the damage done to Rome in the wake of the Visigoths who sacked the city in 410. A larger number of buildings still had to be repaired and rebuilt, one of the most famous of which is the Maria Maggiore (St. Mary Marjor). The mosaics here depict the growing influence that the veneration Mary was having on Catholicism.
