Question:
Is it wrong to marry someone outside of our assembly?

Answer:
1 Corinthians 7:39 says that: “The wife is bound by the law as long as her husband liveth; but if her husband be dead, she is at liberty to be married to whom she will; only in the Lord.”

It is most important to marry “only in the Lord,” only someone who wants to live his/her life for the Lord. This does not mean to marry just someone who is saved, but to marry one who believes the way you do about the Lord’s things, including how the Lord shows us in His Word to gather unto Him with other Christians.
It may not be “wrong” to marry someone “outside of the assembly,” but it would be wrong to marry someone who does not seek to please the Lord, nor seeks to put Him first in his/her life. What a denial it would be of the beautiful picture that marriage is (of Christ and His bride, the church), and of the unity of marriage—if one would function in the assembly and the other would not! If one would go to the assembly meetings and the other would go to a church. Also, what confusion this is to the children in that marriage. The Lord asks the question: “Can two walk together, except they be agreed?” (Amos 3:3).