And the LORD went before them by day in a pillar of a cloud, to lead them in the way; and by night in a pillar of fire, to give them light; to go by day and night:
All Commentaries on Exodus 13:21 Go To Exodus 13
Gregory The Dialogist
It was also fitting that a pillar of fire preceded the Israelites as they progressed through the desert during the night and a pillar of a cloud during the day. There is dread in fire but a gentle soothing quality in the sight of a cloud. “Day” is understood to point toward the life of the righteous and “night” that of the sinner. Hence Paul said to sinners who had been converted, “You were once darkness but are now light in the Lord.” The pillar was revealed as a cloud during the day and as a fire during the night since almighty God will appear soothing to the righteous and dreadful to the unrighteous. When he comes at the judgment, he will reassure the former by his gentleness and mildness and cause dread in the latter by the strictness of his justice.