Matthew 6:22-23

This is the whole context of the warning. Verse 23 is a statement; an exclamation, not a question.

22 The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light.

23 But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness!

24 No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.

thelord's_pearl said:

The eye is the lamp of the body. If your eyes are good, your whole body will be full of light. But if your eyes are bad, your whole body will be full of darkness. If then the light within you is darkness, how great is that darkness!
Why did Jesus have to say that extra sentence I underlined above and bolded. Thank you

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This reminds me of something James said "15 Such “wisdom” does not come down from heaven but is earthly, unspiritual, demonic." There is a wisdom to the "dark side". I talk about this in episodes 2 and 3 of this podcast -- ‎Zaph Naph Paaneah on Apple Podcasts

thelord's_pearl said:

The eye is the lamp of the body. If your eyes are good, your whole body will be full of light. But if your eyes are bad, your whole body will be full of darkness. If then the light within you is darkness, how great is that darkness!
Why did Jesus have to say that extra sentence I underlined above and bolded. Thank you

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He was making an analogy between our bodily vision and that of the soul. In the underlined sentence He was referring to the eye of the soul, i.e. the mind. If then our mind is not illumined by godly thoughts (the light within), but dwells on worldly or evil things then how great is the darkness of our soul!

A similar analogy applies to the injunction to put oil on our head and wash our face. Head of the soul is again the mind, which we need to gladden* with the oil of the knowledge of God, His commandments and with godly thoughts. Then our words and actions, will be pure and this is the washing of our face (our outward way of life).

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* When the Jews fasted they put ashes on their head and looked sad and miserable so as to make a show of their fasting. This is not how we should fast.

ZNP said:

This reminds me of something James said "15 Such “wisdom” does not come down from heaven but is earthly, unspiritual, demonic." There is a wisdom to the "dark side". I talk about this in episodes 2 and 3 of this podcast -- ‎Zaph Naph Paaneah on Apple Podcasts

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Think about Hitler, or Stalin, or Bundy, or Pol Pot. They all had a wisdom, a "light within them that was darkness, and how great was that darkness.