For those interested in early Christianity and the seven ecumenical councils, a very important resource is that of the 5-volume work of Karl Joseph von Hefele, a Roman Catholic historian, A History of the Councils of the Church From the Original Documents. While one can purchase these volume "in print" (which I did a few years ago--rather good investment), they are also available on Archive.org:
Volume 1 (up to AD 325)
Volume 2 (AD 326-429)
Volume 3 (AD 431-451)
Volume 4 (AD 451 to 680)
Volume 5 (AD 626-787)
Volume 1 (up to AD 325)
Volume 2 (AD 326-429)
Volume 3 (AD 431-451)
Volume 4 (AD 451 to 680)
Volume 5 (AD 626-787)
