The answer is: heretics
Heretic is a term given to the people who are not willing to accept religious teachings. At that time, the Spanish empire made Catholicism as a national religion and enforce its citizens to follow the belief. If they refuse they could either jailed or executed.
At that time, atheists were not the only one that became the victim of Spanish Inquisition. Even though technically they believe in God, people with different religion such as Jews, Muslims, or Buddhist also became the target for execution.