1 John 4:1 - "Do not believe every spirit, but prove whether the spirit is from God, for many false prophets have gone out into the world". That was written 2,000 years ago, before paganism and Rome got entwined with Christianity, and before democracy came along to make people think that human rights gives them a right to insert personal desires into church doctrine where a majority agrees. So it is even more relevant in the present day, that most people have been led far, far away from the authentic spirit of God, especially with Television having had a huge influence on public perception and belief of what Christianity is all about.
Jesus said "let no man separate what God has joined together". You are joined on the foundation of love. Beliefs are not greater than love, but are a reflection of knowledge. If you have a spiritual connection through love, it shows you have similar values in heart. Then why he believes differently to you is not a matter of differing values, but of knowledge (experience, teaching, information). You obviously understand the history of Jesus well, and why it is significant in the world. So consider, if he was to understand as you do, would he be Christian or not? There is your answer, and if so, then prayer can connect you with God, to know His will for how you should act, what you should say, to sow seeds of knowledge in his mind and to show forth the light that will draw him to Christ.
1 Corinthians 7:12-16 addresses this precisely. It is in fact not your decision to make, but his, and you should not try to influence him in that decision. Rather, what happens as someone becomes a believer, is that The Holy Spirit through them begins the war over the soul of the partner, and it is a spiritual war that you might not realise is going on. But that partner has to make choices about whether he is comfortable with who you are becoming, and it will produce that either he will hate The Holy Spirit that is in you and will desire to leave - then you must not forbid it (1 Corinthians 7:15). But if he continues to love you even when you have holy spirit, then he is one who loves The Holy Spirit and eventually The Lord is able to bring him into a knowledge of salvation too (1 Corinthians 7:16).
This is the nature of life in faith! Your life is not yours to live however you want, but it is a living sacrifice (Romans 12:1) so that through your patience and faithful love, He is able to save souls as they come to enquire of you, recognising that you are a reliable witness of The Holy Spirit.
It is unusual that you should consider destroying a thing that is only good, especially where it will cause him to forever hate God for having irrationally taken his partner away and ruined his life, when he had been supportive all along! .. So beware the wolves who come in sheep's clothing, who inwardly seek to devour.
.. unless you can provide some rational reason for why you should leave him, I say do not allow fear to be your master. The one who fears has not been perfected in love, but perfect love drives out fear (1 John 4:18).
Dear Lord God, I thank you that our sister can so freely ask for advice, and we thank you that you have kept her safe from following the deceiver's lies to destruction. I ask that you will give her greater wisdom to discern those that are good for her from those who would ruin her, and to recognise that a badge does not necessarily denote honour. We ask for your blessing upon this family, that love will reign to your glory. We ask that you will stir up in our sister a real desire to find that intimate relationship with you so that she can always look to you for advice, to guard her from making mistakes through trusting the advice of fallen creatures. I pray in Jesus' name that Your Holy Spirit will succeed to save the souls of this couple as you know them to be well matched and capable of working things out together through love, loyalty and service to each other's needs. Amen.
