There were 34 popes in the first 300 years of the Church's history. The 34th, discussed today, is Pope St. Marcus (or Mark). The Liber Pontificalis lists him as a Roman, the son of a man named Priscus.
Marcus was apparently active in the Roman church for many years, living through one of the most trying times in Christian history. In 314, Constantine wrote a letter to "Bishop Militiades and Mark" discussing the problem of the Donatists and the synod in Arles dealing with them. For the emperor to write a letter to a bishop and another, we can assume that this Mark, most likely the same man, was of importance to the Church. He was probably already a major player in Church government before that, either as a priest or as first deacon.
However, it was many years before he came to the forefront of the Church, Sylvester having been Bishop of Rome for over twenty years. Marcus was consecrated on January 18, 336 and died of natural causes on October 7 of the same year. Most evidence shows him to have been elderly before his consecration.
For the most part, peace reigned in Christianity. The notable exceptions were the Novationists, those who could not forgive those who lapsed during the persecutions and who were still a respected presence in Rome. The other exception were those who, in the East, followed the teaching of Arius, who insisted that Jesus was begotten of the Father, not eternal, but created in time. This implied that He was lesser than God the Father, in a similar way that the Jehovah's Witnesses see Him. Of course, this lead to political intrigue and disruption through the years. there is no extant evidence of how Marcus thought or dealt with these problems, but, having been in the Roman Church so long, we assume that he thought similarly to the others in Rome and did not agree with Arius.
Marcus began the building of two basilicas, one in the area of juxta Pallacinis, within the walls, which became the titular church of San Marco. The second is outside the walls between Via Appia and Via Ardentina, over the catacomb of Balbina. This is where this pope was initially buried.
Constantine is credited with gifting land and liturgical furniture to the churches, although, during his reign, he tended to side with the Arians, probably in an attempt to coalesce the various beliefs into one.
Under the administration of Marcus, the compilation of lives of the martyrs and bishops began. This is a large step forward in the retaining of history of the Faith.
Marcus is said to have written a constitution granting the bishop of Ostia the right to consecrate new popes. This was verified by St. Augustine a few years later, but may have been a codifying of what was already a custom.
In a reign that lasted only ten months, Pope Marcus succeeded in building churches and developing a compilation of history for which the Vatican became well known.