I would like to know if my Grandfather's reaction can be justified.
I would like some advice about what I may say or write to him.
I feel really sad about their reaction of me changing my views that God does not exist and have stopped practicing.
Why can't they respect my decision and be happy for us?
I feel that I am being pressurised into soomething I do not feel comfortable with just to please them.
My fiancee though baptised does not practice and nor does his family. I feel more supported by them than my own (Catholic) family.
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I will try to answer these for you, in charity, as best as I can.
1) Yes, your grandfather's reaction can be justified. Since you are Catholic, your grandparent's have every reasonable expectation that you will receive the Sacrament of Matrimony in a Nuptial Mass. For you willing not to do so, from his perspective, is an assault to your faith. In spite of what you may think, he is not required to attend your wedding if he reasonably believes that what you are doing is sinful.
2) All you can do or say is this---Grandpa, I am sorry that I have hurt you with my actions and attitudes. However, I must be honest with you and myself. I know longer believe in God at all. I know this is tremendously upsetting to you. Perhaps, sometime we can sit down together, you and I, to discuss why I have come to this conclusion. Since I know longer believe in God, I can not see any value or purpose in having a Nuptial Mass. I also understand that, based on my decision to not marry as a Catholic in a Church, you will not be attending my wedding. While it hurts me deeply because I love you so very much, I understand your position. It would be unjust of me to ask you to compromise your beliefs and values to ease my feelings. I do love you. Your Granddaughter.
3) Why do you feel sad about their reaction? Do you really think that they are just supposed to be happy with everything that you do, even knowing that you are damaging your soul? Imagine how they feel. They know that they could loose you forever to Hell. How horrible that must be.
4) The verb respect means to show regard or consideration for or to hold in high esteem. As a Catholic, we are not called to show regard or consideration for any individual who knowingly rejects Our Lord. In fact, we are called to do just the opposite. Our Lord has commanded us, in love, to confront the evil head on and pray for the best. This is called charity. Your Grandfather is simply doing what he has been commanded to do--confront your behavior. How in the world can he be happy for you when he knows that what you are doing is jeopardizing your soul?
5) Having said all that, you should under no circuмstance get married in a Nuptial Mass. You are right to resist this pressure. While your grandfather only wants what is right, you would be wrong to receive the Sacrament. I'm not certain it would be valid anyway since you don't believe. However, you would only be doing so to make your grandparents happy. In this case, this would be a mockery of God and His Sacraments.
6) Of course you are more supported by your fiance's family than by your own. Non-believers and non-practicing Catholics always "feel" in good company around those with similar lifestyles.
My personal comments: I think you grandfather sounded quite concerned yet loving. I hope that you two are able to work through this eventually. I don't mean that you should insist that they "respect" you and your fiance's lifestyle. I also don't mean that you should just "go along" to make the peace. This warrants a discussion. I am the only Catholic in my family. I recently had to attend a wedding that I otherwise would not have attended. I did so only because of my Mother. My not attending would have caused a much deeper offense. Your grandmother has chosen this option. Your grandfather has not. I will keep you in my prayers and hope you will eventually find your way home.
Peace