Christians of that time was a reference to the 4th century. The historical record indicates that Mary’s perpetual virginity was a widely held belief at that time. As I wrote, I cannot point to any documents prior to the 2nd century.
These “sources” Catholics like to imply early Christians believed concerning Mary as well as the early life of Jesus are actually gnostic writings which appeared much much later (tenth century or later). See:
http://www.earlychristianwritings.com/infancythomas.html
http://www.earlychristianwritings.com/infancyjames.html