Lewis is thinking of the Last Judgment (see second Bookmark for page 86 above). Other passages that relate to Lewis's comment here include the following.
"For other foundation can no man lay than that is laid, which is Jesus Christ. Now if any man build upon this foundation gold, silver, precious stones, wood, hay, stubble; Every man's work shall be made manifest: for the day shall declare it, because it shall be revealed by fire; and the fire shall try every man's work of what sort it is. If any man's work abide which he hath built thereupon, he shall receive a reward. If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire." (1 Corinthians 3:11-15, KJV, emphasis mine)
"Neither is there any creature that is not manifest in his sight: but all things are naked and opened unto the eyes of him with whom we have to do." (Hebrews 4:13, KJV, emphasis mine)