What is the connection between agriculture and spiritual understanding in the Bible?
Because Israel was an agrarian society, everyday agriculture became a ready picture of spiritual truth. A yoke joined two oxen to pull in unison, so Paul's command not to be unequally yoked with unbelievers drew on that image. Deuteronomy's law against sowing a vineyard with diverse seeds reinforced the same separation principle.
Les explains that as you read the Old Testament, Israel was a rural or agrarian society, still living back in antiquity when animals were used as beasts of burden. Therefore there is a lot of correlation between the agriculture of the day and spiritual understanding. So when Paul says, “Be not unequally yoked,” the word yoke immediately brings to mind beasts of burden out in the field plowing. A yoke put two oxen together so they could pull in unison. Les then takes it to Deuteronomy 22:9-10, part of Israel's law, where Israel was told not to sow a vineyard with diverse or different seeds, lest the legitimate grapevine and the fruit of the vineyard be defiled. The admonition is still the same today: be not unequally yoked together with unbelievers.