St Soter was Pope for eight years between 166 and 174 although, as with all early Popes, dates vary up to five or six years either way. His saint day is on 22nd April along with St Caius, a third century Pope.
Little is known about Soter it is thought he was born in Fondi, in Italy, and that his Greek name, meaning Saviour, was given to him by his Greek father. The only real information we have about him stems from a letter written to him by St Dionysius of Corinth that is quoted by Eusebius in his "Ecclesiastical History".
"From the beginning it has been your custom to do good to all the brethren in many ways, and to send alms to many churches in every city, refreshing the poverty of those who sent requests, or giving aid to the brethren in the mines, by the alms which you have had the habit of giving from old, Romans keeping up the traditional custom of the Romans; which your blessed Bishop Soter has not only preserved, but has even increased, by providing the abundance which he has sent to the saints, and by further consoling with blessed words with brethren who came to him, as a loving father his children."
"Today, therefore, we have kept the holy Lord's day, on which we have read your letter, which we shall always have to read and be admonished, even as the former letter which was written to us by the ministry of Clement."
(Euseb., Hist. Eccl., IV, xxiv.)
It is this letter which gives rise to his moniker the Pope of Charity.
It is also believed by some that Soter declared that marriage was only valid as a sacrament when blessed by a priest. Some sources also state that he formalised the date of Easter as always being on a Sunday. Finally it is also said that he forbade women (sometimes translated as nuns or monks) to touch the altar cloth or burn incense in a church. There seems to be no evidence for any of these decrees other than tradition.
Soter died sometime around 174 and tradition states that he was martyred and is buried in the cemetery of Calixtus where there is a memorial chapel dedicated to him.
Anicetus - Pope - Eleutherius
http://www.cfpeople.org/Books/Pope/CFPtoc.htm - Popes through the ages by Joseph Brusher