1 John 1:5-2:2

v5 Provides is the ground, the foundation for what follows in verses 6-10 and, in a sense, for the whole rest of the letter. Findlay said, “Nothing has
ever given such relief to the human mind as the announcement of the simple truth of this verse.” The figure of light that John used to describe God emphasizes both His ability to reveal and His ability to deal with what thelight of His holiness reveals. For John, God being light states God’s infinitely superior ethical being and His illumination of man’s ethical deficiencies.
God’s light reveals the cancer, to provide the healing surgery and therapy.