This is often used as an excuse to engage in or remain in a sinful state. Sin is still sin. Being the only Christian in a person’s life is not a reason to sin. (In America, this is not meant as being the only Christian they have access to. Obviously, Americans have access to churches, Bibles, and Christians all over.) Staying in sin to stay in their life is actually the exact opposite of what will win them. The unsaved person will not understand you are staying in their life or in that situation in order to win them. They will see that the sin you are involved in doesn’t matter to you. They will see that there is not a difference between being a Christian and not.
God commands us to be separate. Be Holy. Be apart from the world. No where in the Bible does God say to do anything because you are the only Christian witness in someone’s life. We are to preach the Gospel to every creature. We are to make disciples. We are to teach. We are to live so that by our walk, countenance, and conversation, men may know that we are different. But no where does God say “You that are the only Christian in someone’s life, take special care to stay in their lives – even bending and breaking my rules if necessary.” God doesn’t say that.
God says we will win them by being holy. It will be our good works that bring glory to God. It will be our faithful witness.
Matthew 5:16 “Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven.”
1 Peter 2:12 “Having your conversation honest among the Gentiles: that, whereas they speak against you as evildoers, they may by your good works, which they shall behold, glorify God in the day of visitation.”
1 Peter 3:1-2 “Likewise, ye wives, be in subjection to your own husbands; that, if any obey not the word, they also may without the word be won by the conversation of the wives; While they behold your chaste conversation coupled with fear.”
God makes several things very clear in the Bible: be holy, walk in the Spirit, pray, flee temptation, serve the Lord, praise Him, study His Word and the list goes on. It is certainly not wrong to be the only Christian in someone’s life. But it should never be the reason for something. It is not a litmus test about where you should work, or live. It is not a test of whether you should be someone’s friend, or build a relationship, or keep one. It is not a test at all. God uses His Word, prayer, and Godly counselors to direct us. Use those. Pursue after God. Talk to His people. Be wise.