When Jesus first began to perform miracles, many believed on Him because of what He did. They believed because of what they saw, rather than because of who Jesus was. But verse 24 tells us that Jesus did not commit Himself to them, because He knew all men. Jesus didn't trust them because He knew their hearts. He didn't need anyone to tell Him what someone was truly like because He knew everything about them, even their thoughts.
When the Spirit of God is moving, people sometimes get caught up in the flow. Then when things begin to cool off and times don't seem to be so thrilling, they back away from God and sometimes lose their faith.
When we decide to follow the Lord, we cannot go by our feelings, or even by sight. We must live by faith. It is wonderful when the Spirit of God is moving and miracles are being performed. However, we must remain steadfast in our faith, even when there doesn't seem to be much happening. God is still God and we love and serve Him because of who He is, not because of what He can do for us. We must remain faithful to Him regardless.