How does Karma work for a person who has converted to Hinduism?
Karma is what you had done and what you are doing in your life time. So Karma will be there from the very next moment of your birth. There is nothing to do with Hindu, Muslim or Christian.
The Karmaphalam, Fruit(Reward) for your Karma will be awarded to you in your life time, may be in your next life too. Whether you are or were a Hindu or Muslim or Christian won't be considered in this.
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