The first sign of Christianity occurred in SG-1's third year of operation when they journeyed to a planet inhabited by medieval Christians. Dr. Jackson speculated the original inhabitants must have been transported through the Antarctic Stargate.
The medieval Christians of this world were under the dominion of the Goa'uld Sokar, who would send his Unas as dark emissaries each season to capture a number of locals and take them back to it's master, presumably to become hosts or be sent into the fires of Ne'tu.
The medieval Christians were led by the Canon, a man of ultimate authority who used a weather control ring to show his dominance and superiority, believing only he had the power and blessing to condemn selected people to return with the demonic Unas.
With the arrival of SG-1, the team managed to destroy the Unas and, out of necessity, kill the Canon, who had been possessed by the Unas' desperate symbiote, leaving the medieval Christians to hopefully live a better life for themselves and their higher power.
Demons - SG-1 discovers the first hint of Christianity, and a threat that continues to keep them subservient to the Goa'uld.