Can two walk together, unless they are agreed?
–Amos 3:3 (NKJV)–

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 (NKJV)–

A double minded man is unstable in all his ways.
–James 1:8 (KJV)–

How long will you falter between two opinions?
If the Lord is God, follow Him; but if Baal, follow him.
–1 Kings 18:21 (NKJV)–

—

Dr. Ernest Pickering, Biblical Separation (Regular Baptist Press, 1979).

Dr. John R. Rice, The Unequal Yoke (Sword of the Lord, 1946).

—. Come Out or Stay In (Thomas Nelson, 1974).