Light is a short term irritant to the eyes. Once the eyes are adjusted to the dark. And if the eyes are no longer an issue to the light, when the eyes are used to the dark, there is an issue of losing sight where the darkness becomes great.
The principle is the same for the soul. The light is the truth of God’s word. God’s word is an exasperation to anyone who is in darkness, (evil) or has adjusted themselves to it. Believing in Satan’s lying deceptions. When the light of truth no longer bothers a person consciously while in evil, they have lost their conscious sensitivity to the truth. And have embraced lies as truth.
Hence, those who are in darkness are easily annoyed by those who walk in the light. And have no fellowship with them. For they have nothing in common. Having opposite values.
One can grow to love the light. Where worldly pleasures what are temporarily unhealthy and exhilaratingly thrilling, grow to be strangely uninteresting.
Therefore, whoever claims to have fellowship with unbelievers is not a believer walking in the light. But a person who is in darkness. Walking in darkness as unbelievers do.