How do we know the doctrine of separation from unbelievers is true?

The doctrine of separation from unbelievers is affirmed by numerous scriptural admonitions, particularly in 2 Corinthians 6:14-18, where Paul explicitly instructs believers to avoid forming deep connections with those who do not share their faith in Christ. This call is based on the foundational belief that believers possess a new nature that differs radically from that of unbelievers. The Apostle addresses the issue by asking rhetorical questions about the compatibility of righteousness with unrighteousness, light with darkness, and Christ with Belial (Satan). Furthermore, the Old Testament provides further support for this doctrine, as seen in Deuteronomy 22:10, which illustrates the practical implications of mixing different kinds. Hence, the doctrine of separation is importantly validated by the authority of Scripture and is a key aspect of preserving the integrity of the Christian faith.