Is it a mortal sin to date someone who is legally separated but not divorced? He is currently going through the process of divorce. He was not married in a Catholic church.
It is sinful to date anyone who is still married. Unless their marriage has been declared null by the Catholic Church, they must be presumed to be still married whether they are separated or divorced. The Catholic Church does recognize the marriages of non-Catholics to each other as valid. No state has the power to dissolve “what God has joined together.”
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