"Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?" (II Corinthians 6:14).
This is one of the definitive statements in Scripture on the doctrine of Christian separation. Not only should believers refrain from practicing evil teaching and error, they should not join in any formal association with those who do such things, nor should they enter into a binding relationship of any kind with non-Christians.
There may be a question regarding the full scope of this prohibition, though it probably would not apply to civic clubs, professional societies and such like, with no religious connotations. The context of this verse implies an association of Christians with pagan idolaters, compromising God's word with the immoral pantheism of the Greek religions.
The "unequal yoke" seems, therefore, to be one involving an actual "fellowship" and "communion" in some kind of religious or quasi-spiritual union with unbelievers, and this is forbidden, for how "Can two walk together, except they be agreed?" (Amos 3:3). In the modern scene, ancient Greek pantheism has now become one form or another of evolutionary humanism. Thus the prohibition would at least apply to membership in secret lodges or fraternities with a pseudo-religious structure and purpose, as well as membership in liberal churches or cults in the so-called "New Age" orbit. It clearly must also include marriage or partnership or other formal unions with individuals who, as unbelievers in Christ, are either knowingly or unknowingly affected by such pagan beliefs or practices. Instead of such an unequal yoke, we should be joined only to Christ and His followers, "For," said He, "my yoke is easy, and my burden is light" (Matthew 11:30). HMM