We walk by faith, not by sight . . . while we do not look at the things which are seen, but at the things which are not seen. (2Co 5:7 and 2Co 4:18)
The Christian life is a walk. It is comprised of day by day steps from where we are to where the Lord wants us to go (both spiritually and geographically). This walk is undertaken by faith, not by sight. "We walk by faith, not by sight."
Walking by sight is the natural manner by which human beings walk. This is true both for literal walking, as well as for taking the proverbial journey through life. When engaged in physical walking, people rely upon visual data (along with input from other human senses: like sound, smell, and touch). Likewise, as the unredeemed are engaged in their trek through life, they set their course and proceed by that which their natural abilities provide. We who know the Lord Jesus Christ cannot walk in this manner in His kingdom. We must walk by faith, by depending upon our Lord, His word, and the work of grace by His Holy Spirit. Spiritual progress is made "while we do not look at the things which are seen, but at the things which are not seen."
So often, things are not as they appear to be in the natural. Consider Joseph being sold to slave traders by his jealous and deceitful brothers. It did not look like Joseph was being groomed to be Prime Minister in Egypt. Think of Pharoah and his army closing in on Israel, as they were trapped beside the Red Sea. It did not look like Israel would be delivered, while the Egyptian army would be destroyed. Remember young David standing before gigantic Goliath. It did not look like the giant would be defeated, while David would enjoy a thorough victory. Only eyes of faith could really appreciate what was actually happening.
The cross of our Lord Jesus is undoubtedly the greatest example of things not always being what they appear to be. As Jesus hung upon the cross, it appeared that godless men had defeated the most godly man that ever lived. "Jesus of Nazareth, a Man attested by God to you by miracles, wonders, and signs which God did through Him in your midst . . . Him . . . you have taken by lawless hands, have crucified, and put to death" (Act 2:22-23). Yet, in fact, God was at work, preparing a resurrection victory over sin and death for all who would believe. "Whom God raised up, having loosed the pains of death, because it was not possible that He should be held by it" (Act 2:24).