Avoid the unequal yoke

Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness? And what accord has Christ with Belial? Or what part has a believer with an unbeliever? - 2 Cor. 6:14-16

You have heard of the boys who found two linnets (finches) in the field. They brought them home and put them in cages hung on either side of their canary. The mother asked the reason for this, and they said, "WeIl, you see, we got these young, and we have hung them by the side of the canary, so that they will be accustomed to listening to the canary sing, and so instead of learning to chirp like a linnet, they will learn to sing like the canary." The mother did not say anything, but shook her head, and, later on when they came in, they exclaimed, "Why, Mother, listen! Our canary is chirping like a linnet!" That is the way it works. That canary did not get its song back again until they had covered it up for some days, and then when they put it out in the bright sunshine and took the covering off, the little thing began to sing once more. If you are going to count for God you must avoid the unequal yoke. If not, you are going contrary to the Word of God and you cannot expect blessing.

- H. A. Ironside, Commentary on 2 Corinthians

"I am the light of the world" John 8:12