The light shines in the darkness, but the darkness has not understood it.
The purpose of light is to shine and thus dispel darkness, whether in the physical or spiritual realm. It is to be noted that “light and darkness are opposites, but they are not opposites of equal power. Light is stronger than darkness; darkness cannot prevail against it.” (F.F. Bruce)
Up to this point John has only used the past tense, but now he switches to the present, “And the light shines in the darkness.” The light continues, even now, to shine and get rid of the darkness.
“understood” in the NIV is translated as “comprehended” in KJV and means to lay hold of so as to make it one’s own, either physically or mentally.
If it is meant as mentally, then this could be translated as “understood” (which is the manner NIV translates it) and would refer to those men who are walking in darkness and don’t understand or comprehend the truth. If physically, then it could be translated as “overtook” or “overcame” and would refer to the evil environment in which we find ourselves. In this case, the meaning of the second half of the verse is that the darkness did not overcome the light: the light won! Notice the change of tense within the verse. John starts with the present tense to indicate that the light is even now shining but switches to the past tense when speaking of the overcoming of the darkness by the light. The light has already overcome the darkness. When did this happen? At Calvary with the great victory won for us forever by the light. (1John 2:8) Someone once said that “Not all the darkness in the world can extinguish the littlest flame.” (qt’d in Barclay) What Jesus brought to us was not a small flame, though, but a blazing sun, blasting darkness into non-existence.
1 John 2:8 Yet I am writing you a new command; its truth is seen in him and you, because the darkness is passing and the true light is already shining.