Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? (2nd Corinthians 6:14.)
There are two ways to view this scripture. 1) You were already received the baptism of salvation and then found a significant other who isn’t saved or 2) Along the way, one got saved and the other didn’t.
The Lord has called us to be separate from the things of the world and those in it. Why? Because of your love for the unbeliever, you would be swayed to doubt the word of God and harden your heart.
Let’s view this scripture from both angles. The basic premise is that: “You can’t force someone to believe in the Lord and the power of His word.” The more force you use, the more alienated to the word they become.
Instead of “hiding” your faith, display your faith in action for all to see. And over time, they may believe that “God is and He is a rewarded of those who diligently seek him (Hebrews 6:11).
For instance, every time something comes up that you have no answer for – pray about it and the answers you are hoping for. And when God provides you with the answer to your prayer, acknowledge that it was Him. Praise Him and do it in front of your unbelieving partner. Eventually, you will have a stack of “Answered by God” prayers that you can go down a list of things that the Lord God answered as proof (over the years) of the Lord’s existence and that He is the supplier of all of our needs (Philippians 4:19). That the Lord God answers prayers and He will help all of those who call upon His name. He will save us from all our adversities (Psalms 34). It’s not like a bashing-over-the-head with God but more like a someone looking over your shoulders discovering God for themselves.
For several years, I dated and was engaged to Ebony, who didn’t really know the faithfulness of God and never had a man who knew the Lord God by faith in action. As we got into situations where our electricity was about to be shut off or others situations where we were going to be put out because of having no money for the rent or even not having money for our daily bread, through my prayers and my faith in God, the Lord God showed up and met our needs right on time. From my eyes the Lord provided like I believed He would. From what Ebony could see was that there was a God who is merciful and who answers prayers of the believers. We have since moved on with our lives but I believe that Ebony learned how to use her faith to believe God for answered prayers. Praise the Lord!
To the rest I say this (I, not the Lord): If any brother has a wife who is not a believer and she is willing to live with him, he must not divorce her. And if a woman has a husband who is not a believer and he is willing to live with her, she must not divorce him. For the unbelieving husband has been sanctified through his wife, and the unbelieving wife has been sanctified through her believing husband. Otherwise your children would be unclean, but as it is, they are holy. But if the unbeliever leaves, let it be so. The brother or the sister is not bound in such circumstances; God has called us to live in peace. How do you know, wife, whether you will save your husband? Or, how do you know, husband, whether you will save your wife? Nevertheless, each person should live as a believer in whatever situation the Lord has assigned to them, just as God has called them. This is the rule I lay down in all the churches (1st Corinthians 7:12-17 NIV).
By the example I shared about Ebony, it was through my love for her that I was willing to share and invite her into my faith arena with God to bring us on the same level not only through our relationship with each other but also with the Lord God. Bringing someone on the same faith level as you doesn’t always work. Most of time, the unbeliever will hide in the safety of their unbelief or convert the believer to their ways concerning certain elements of the faith walk. Therefore as the believer, you must always be able to give an answer to what you believe and you believe it. In order to do that, you have to be continually submersed in the word so that whenever doubt rears its ugly head, you have an answer.
But in your hearts revere Christ as Lord. Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have. But do this with gentleness and respect, keeping a clear conscience, so that those who speak maliciously against your good behavior in Christ may be ashamed of their slander (1st Peter 3:15-16).
In the second part, it just so happened along the way, one got saved and the other (my partner) didn’t. What should I do? How can two walk together unless they agree? Since you got saved and they didn’t, it wasn’t because of the Word of God – it was on account of their unbelief. You can’t force someone to believe, all you can do is show the love of God through your actions, attitudes and your words until they began to believe. Love will keep you together for a time, and discovering why the other partner didn’t believe is going to the root of the problem. Yet, still it may not convert them into a believer. If they are willing to continue to love you, then you must continue to grow strong in the Lord and the power of His might. For one day, especially if you have prayed about it, the Lord will bring into your relationship a situation (or two) which will prompt your (unbelieving) partner to ask you to pray for something or another. Just know that it isn’t going to get them saved right away, it just shows that they believe you have a connection with God that produces results. As the years progress, you will find out that they believed in God but didn’t know how to outwardly express it. To God belongs the Glory.