The two pillars [Exodus 13:21-22] actually are one. When night comes, the cloud becomes the fire. But when day dawns, the fire becomes the cloud. Nevertheless, the fire and the cloud are one.
In typology, the cloud signifies the Spirit. As He reaches us, the Spirit of God is like a cloud. The fire here signifies the Word of God, which is a light to us. Hence, the instant, living guidance that God gives us comes either through the Spirit or the Word. When the sky is clear, He is the cloud. But when the sky is dark, He is the fire. When the Lord leads as a pillar of fire by night, the light from this pillar causes the night to become day. In this way the children of Israel could journey day and night.
In the Christian life there should be no difference between day and night. Actually, we Christians should not have any night in our life with the Lord. Instead, our night should turn into day. If we have a night that does not turn into day, we are defeated. Whenever we are in darkness and are sleeping, we experience a night in our life as Christians. But when we call on the name of the Lord, we are enlightened, and our night becomes day. (Witness Lee, Life-study of Exodus, msg. 27)