This is a more common concern when people are considering putting their faith in Christ than you might think, and you need to be so careful with your answer. The question involves a lot of feelings and clearly the person loves their spouse a lot. You also need to be very aware of where this person is in their journey to Christ. If they have been reading the Bible and/or coming to church you can talk to them using the Bible, but if they don’t have that knowledge, it may be more helpful to simply give them reasons to move forward with Christ.
The first thing you need to settle is what you yourself think about this issue. Yes, the Bible warns us not to be ‘unequally yoked’ with an unbeliever (2 Corinthians 6:14), but if a person is already ‘yoked’, that should not hold them back from Christ.
In fact, Paul had already written earlier to the Corinthians and told them just the opposite—that a believing wife or husband should stay with their spouse, “for how do you know, wife, whether you will save your husband? Or how do you know, husband, whether you will save your wife?” (1 Corinthians 7:12-16).
The most likely way that their spouse will turn to God is for them to turn to God and bring the aroma of the knowledge of Christ into the relationship. So as you speak with this person, affirm and encourage their love for their spouse. Do not dismiss their concern. It is real. But give them the illustration of the procedure on an aeroplane if the oxygen masks are deployed: “First, affix your own oxygen mask and then you will be able to help others.”
Help them see that the reality of God in their life is what will show their spouse the reality of God.
There are a lot of stories around where the husband or wife has become a Christian and their spouse has seen the difference in their lives—how Jesus has changed them for the better. This has resulted in the spouse also becoming a Christian. Encourage them to do a Google search on it and find heaps of stories.
They want to find forgiveness and a hope for the future, wouldn’t they also dearly love their spouse to find this same forgiveness and hope? And when they think about their hope for their eternal future, if they never come to Christ and receive it, then their spouse definitely never will!
But they should also know that there is no guarantee that their spouse will follow their decision. They have to choose to put God first in their lives, even if their spouse doesn’t.
As they put their trust in Jesus and start a relationship with him, they can know that God loves their spouse even more than they do. He revealed himself to them, they can trust that he will reveal himself to their spouse as well. Encourage them to start praying for their spouse by name.