What were the pagan gods the Galatians worshipped before they knew God?
Before knowing God, the Galatians were steeped in pagan darkness and rendered service to pagan gods that by nature were not gods at all. Those objects of worship were nothing but wood or stone, lifeless idols rather than true deities.
Les explains that the Galatians had been steeped in pagan darkness and had done service to pagan gods. When everything is boiled down, however, those gods were not gods at all; they were nothing but a piece of wood or stone. The context does not name specific pagan deities. Les also notes that Paul's converts came out of paganism and immediately faced persecution, and he contrasts their willingness to die for the faith with the question of how many professing Christians today would remain under similar pressure.
Howbeit then, when ye knew not God, ye did service unto them which by nature are no gods.