Do not be unequally yoked together with unbelievers. For what fellowship has rigtheousness with lawlessness? And what communion has light with darkness?
...what is the "agreement" between light and darkness?
The answer is AUTHORIZED POWER.
From the beginning [Genesis 1:1-4], God established the AUTHORITY of LIGHT over darkness. And by doing so, He established the "unequal" setting of the two.
When God saw that the earth was dark, He said "Let there be light'
God saw that the "light' was good
He divided the light from the darkness
By speaking light, darkness had no chose but to submit. Darkness had no power over light.
According to The Gospel of John, "darkness did not comprehend(understand, overpower) the light" [John 1:5].
"You are the light of the word..."
In other words, Jesus is saying that 'you are the authority in the world', ' you have the power over darkness' and most importantly, ' you can change situations, circumstances, atmoshperes'. John says that the light is also life. So in a dark-dead world we are to be the light-life in this world. We are to bring the life that Christ came with.
Darkness can only have the power if and when the light doesn't shine. When the sun rises, the night leaves and the day appears. Turn a light on in a dark room and darkness has to go that the light may shine. There is no bargaining in either of these situations, there is no compromising. The law has been established. The agreement has been made. Authority is just exercised. No matter the size or the dept of the darkness of a room, when light is exercised, light will shine. As much as the law of gravity is true, even so is the law of light.
Now as children of God, we have this law on our side. Jesus Himself exercised this law, 'for with authority and power', He exercised the law of light [Luke 4:31-37]. Jesus knew the agreement between light and darkness, therefore He enforced it. As sons and daughters of God, we have to enforce this law upon any and everything that is not of God. We have that right. We have the authority and power to change any and every situation, circumstance, surrounding that we are put in. We can change those who are around us too, whether it be their conversation, language, attitude, etc.. When we come around, we should shine so bright that darkness have no other choice but to submit.
That's the "agreement". Light says to darkness, ' I have the authority over you'. 'I have the power over you'. 'When I shine, you must flee'. It's the law. It was established by God from the beginning, and it is just waiting to be enforced today.