I heard that praying for forgiveness for dead Christians is forbidden. I would like to know if I can pray to God to forgive my Christian mother who does not want to come to Islam so that she will be one of the people of Paradise. So that even if she dies as Christian then at least she knows that she'll be among the people of the Paradise.
Allah says in the Holy Quran "Indeed, [O Muhammad], you do not guide whom you like, but Allah guides whom He wills. And He is most knowing of the [rightly] guided". (Quran 28:56)
It's clear from this verse that even Prophet Muhammad PBUH has no authority to guide people. Our duty is to ask Allah and pray for them sincerely and constantly to guide her. You have to only ask God for her guidance. Only Allah can change the heart.
Allah knows best.