Today's saint, Pope John I, was born in Tuscany and was elected Pope in 523, when the Church was in, shall we say, interesting times. Italy was ruled by Theodoric the Ostrogoth, who was an Arian, while most of his subjects remained Catholic. As time passed, Theodoric became more and more suspicious of the allegiances of his Catholic subjects, especially since the Catholics had very strong links with the Catholicism of the surviving Roman Empire in the East, where Julian, the Roman Emperor, was prosecuting/persecuting his Arian subjects. The eastern Arians were begging Theodoric to help them.
So Theodoric sent a diplomatic mission to Constantinople, led by Pope John, to plead for better treatment for the Arians. Pope John was successful in obtaining an easing of the persecution, but the great respect and utter enthusiasm with which John was received in Constantinople roused the suspicions of Theodoric. What if John and Julian were plotting together to overthrow him, the great Theodoric???
Immediately upon Pope John's return to Italy, Theodoric had John arrested, and he died there, of ill-treatment in prison, a few days later, in 526.
The life of St. John reminds us of what true tolerance is and isn't. There could be no compromise with Arianism, a dangerous heresy that made the atonement all but pointless, but this did not mean that people who believed in Arianism had lost their human dignity and were fair game for ill-treatment. It is wrong for a state to use the force of the state to enforce uniformity of thought or belief --- even in the attempted service of the truth.
After all, if the emperor Julian can use his armies to enforce Catholic orthodoxy, Theodoric can use his to enforce Arian beliefs --- and do we really want to go there?