Antoninus Pius was known for his peace efforts in Asia, his pardoning of criminals, the protection of slaves, his refusal to execute senators and his generosity. Despite his contributions to the Roman Empire, scholars are uncertain as to the reason he was granted the title "Pius."
Antoninus Pius is one of the few Roman emperors who did not earn his title by birth. Rather, he was adopted by the Roman emperor Hadrian on his deathbed so that he could assume rule of the Roman Empire. Pius was renowned for his loyalty to Hadrian, and it is thought that this might be one of the reasons that he was given the title of "Pius." In fact, he had Hadrian declared a god after he died, a move which was not popular amongst Romans as Hadrian was not well liked.
Pius was respected by the senate, however, for following traditional Roman ideals and for forbidding the execution of senators, which had not been the practice of his beloved predecessor. By all accounts, Pius was a very compassionate and dedicated ruler who gained popularity with the people of Rome. This, also, has been proposed as a possible reason for his being bequeathed the title of "Pius."