"our ancestors." Paul is referring to the time of Moses in the book of Exodus when Israel fled through the Red Sea as the Egyptian army pursued them. The word "our" refers to himself and the Corinthians and is inclusive. (See: [[rc://en/ta/man/translate/figs-inclusive]])
When the Israelites left Egypt, God traveled with them in a pillar of cloud. Alternate translation: "were all led by God who was in the cloud" (See: [[rc://en/ta/man/translate/figs-explicit]])