Regarding your first question, yes, it is a must for a Christian to marry a fellow-believer in order to: 1) Have joy and harmony in your marriage; and 2) Glorify God in your marriage.

There are two verses in the New Testament where God actually commands the believer to marry another believer. In 1st Corinthians 6:14 God says (through the Apostle Paul), “Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with lawlessness? And what communion has light with darkness” (NKJV). These words are pretty clear, are they not? God does NOT want a believer to marry an unbeliever, so the obvious conclusion is you should marry a fellow-believer. If you marry an unsaved person you will not be able to have any FELLOWSHIP with that person, which simply means that you will have NOTHING IN COMMON with them SPIRITUALLY. They will NOT have the desire to obey God’s Word and thus they will NOT live a “righteous life” according to God’s standards of holiness. They are still in SPIRITUAL and MORAL DARKNESS and the Christian is not to have any “fellowship with the unfruitful works of darkness” (Ephesians 5:8). There will be nothing but trouble when it comes to making “moral decisions” if one in the marriage is still in darkness. The prophet Amos put it this way, “Can two walk together, unless they are agreed” (Amos 3:3). The obvious answer is NO! Instead of harmony in the marriage there will be disagreements and conflict. God knows this so with our best interests in mind He commands us NOT to marry an unbeliever.

The second verse I had in mind is 1st Corinthians 7:39 where the Apostle Paul was instructing widows: “A wife is bound by the law as long as her husband lives; but if her husband dies, she is at liberty to be married to whom she wishes, only IN THE LORD.” Unlike the previous verse, where God told believers “who they shouldn’t marry,” here God tells them “who they should marry.” The last three words “in the Lord” convey three thoughts: 1) One should marry a Christian; and 2) One should marry a Christian who submits to Christ AS LORD; and 3) One should marry the Christian that God has created just for them. Regarding #1, it’s not enough to simply marry another Christian, for there are Christians who are “carnal” (see 1st Corinthians 1:1-4). Regarding #2, it’s not enough to marry a “godly Christian” who loves and serves the Lord, for that still might not be “the ONE” that God has for you. Regarding #3, this is the ideal for as I intimated above “God has created just one person especially for you!” You may recall that after God created Adam and then formed a woman from her side, we read that “He brought her to the man.” That is beautiful and precious, for it means that God not only created this woman for this man, but then He presented the woman to the man so they could be united as one in marriage (see Genesis 2:18-23). This means that God will “lead the man and the woman to meet each other” that are meant to be together as one in the sacred bond of marriage. This union is so special to God for it serves to illustrate the bond between Christ and the church, His future bride (see Ephesians 5:22-32; 2nd Corinthians 11:2 and Revelation 19:7-9.

If you take to heart the scriptures we have gone over, you will want to PRAY for the Lord to direct you to A CHRISTIAN WHO HONORS THE LORD in their life and WHO WAS MADE ESPECIALLY FOR YOU! You won’t “force the issue” in any way; rather you can count on the Lord to bring about circumstances in your life where you will meet THE ONE the Lord has for you. You two will then fall in love with each other and desire to serve the Lord together in holy matrimony. Your interests will be in harmony with each other and by the grace of God you will honor and glorify the Lord TOGETHER AS ONE.  (454.1)  (DO)