God’s admonition to His children to keep themselves separate from unbelievers goes all the way back to the days of Adam. From the time that Cain slew Abel God has forbidden his children to intermarry. This prohibition wasn’t limited only to the children of Cain, but also to those who would not believe in His word. He said:
And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God and they shall be my people.
And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty.” (2 Cor 6:14-18)
God lodged his complaint with Noah on this very subject of intermarriage with unbelievers just before he sent in the flood to destroy them in the book of Moses:
“And Noah and his sons hearkened unto the Lord, and gave heed, and they were called the sons of God.
And the Lord said unto Noah: My Spirit shall not always strive with man, for he shall know that all flesh shall die; yet his days shall be an hundred and twenty years; and if men do not repent, I will send in the floods upon them.
And in those days there were giants on the earth, and they sought Noah to take away his life; but the Lord was with Noah, and the power of the Lord was upon him…
And also, after that they had heard him, they came up before him, saying: Behold, we are the sons of God; have we not taken unto ourselves the daughters of men? And are we not eating and drinking, and marrying and giving in marriage? And our wives bear unto us children and the same are mighty men, which are like unto men of old, men of great renown. And they hearkened not unto the words of Noah.” (Moses 8:13-21)
This same pattern of wickedness that we saw in Noah’s day is now happening again.
“And now I say unto you, all you that are desirous to follow the voice of the good shepherd, come ye out from the wicked (the unbelievers), and be ye separate, and touch not their unclean things (their false traditions and their idols) and behold, their names shall be blotted out, that the names of the wicked shall not be numbered among the names of the righteous, that the word of God may be fulfilled, which saith, The names of the wicked (the unbelievers) shall not be mingled (unequally yoked in marriage) with the names of my people.” (Alma 5:57)
And now I say unto you, that the good shepherd doth call after you; and if you will hearken unto his voice, he will bring you into his fold, and ye are his sheep; and he commandeth you that ye suffer no ravenous wolf to enter among you, that ye may not be destroyed.” (Alma 5:59-60)