What did the Galatians do when they did not know God?
Before knowing God, they did service to beings that by nature are no gods, going through superstitious rituals to wood and stone. Those objects had no power except satanic power. Their ritual service was bondage, not a relationship with God.
Les explains that Paul addresses the Galatians’ former condition: when they knew not God, they did service unto them which by nature are no gods. They went through superstitious rituals to wood and stone, which had no power except satanic power. But now, after they have known God—or rather are known of God—they are His, known as a born child. Paul then asks how they turn again to the beggarly and weak elements, desiring bondage. Les identifies this as religion, ritual, service, and trying to appease a holy God, which believers are to have none of.
Howbeit then, when ye knew not God, ye did service unto them which by nature are no gods.