Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness? (2 Corinthians 6:14)
We are in this world with unbelievers. These we are to befriend for the sake of winning them to Christ. Nevertheless we are not to be "unequally yoked" with them. To attempt to be equally yoked with the lawless or those in darkness is to be severely mismatched. It is to be put into a position or relationship that cannot work. Whether in business or marriage, investments or partnerships, two or more traveling in opposite spiritual directions will inevitably destroy that which they are pulling. I do not believe that the Lord is saying for believers not to befriend sinners, but He is saying for us not to be yoked with them. The purpose of friendship with unbelievers is to get them to become believers. It is not to marry or partner. No matter how inviting it may be to you, you are no exception. Only the cross of Christ is able to equally yoke His creation.