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Honest question from a Catholic.
Limited Atonement means that Christ died only for those who were meant to be saved, not for all mankind.
Irresistible Grace means that those who were meant to be saved cannot resist the power or calling of the Holy Spirit.
In a nutshell Calvinism is basically that everything in creation was predestined to happen a certain way and there is very little that we can do about it. If you were called to be saved, you will be saved, if you are called to be damned, you will be damned.