He grew up Catholic.
He recognized the idolatry.
He called the Pope anti-Christ.
He wrote the Institutes.
In addition to the above first answer, listed below are more specific views of John Calvin and Catholicism:
* As like Luther, Calvin believed that the Bible was the final authority on Christianity, not Papal Authorities;
* Calvin formulated the doctrine of "Predestination"'; and
* Calvin denied the Catholic position that only good works can enable a person's salvation.
In addition, Calvin was instrumental in spreading Protestantism throughout Europe.