6 You see, then, my brethren, you see, if you see inwardly, what kind of light this is, of which the Lord says, “He that follows me shall not walk in darkness.” Follow the sun, and let us see if you will not walk in darkness. Behold, by rising it comes forth to you; it goes by its course towards the west. Perhaps your journey is towards the east: unless you go in a contrary direction to that in which it travels, you will certainly err by following it, and instead of east wilt get to the west. If you follow it by land, you will go wrong; if the mariner follows it by sea, he will go wrong. Finally, it seems to you, suppose, that you must follow the sun, and you also travel yourself towards the west, where it also travels; let us see after it has set if you will not walk in darkness. See how, although you are not willing to desert it, yet it will desert you, to finish the day by necessity of its service. But our Lord Jesus Christ, even when He was not manifest to all through the cloud of His flesh, was yet at the same time holding all things by the power of His wisdom. Your God is whole everywhere: if you fall not off from Him, He will never fall away from you.
Source: Tractates on the Gospel of John (New Advent)