Tractate Avodah Zarah 11
Onkelos, the brilliant nephew of the Roman Emperor Titus, converted to Judaism and became a disciple of the Sages. Hearing this, Titus sent a brigade of soldiers to bring him back to Rome. But when Onkelos engaged the soldiers in discussion and showed them the beauty of Torah, they converted to Judaism.
Titus then sent another brigade, instructing them not to speak to Onkelos. But after listening without even speaking, they too converted to Judaism. Finally Titus sent a third brigade and instructed them not even to listen to Onkelos. When they were leading him away, Onkelos placed his hand on the mezuza and inquisitively inquired, “What is that?”
“You tell us,” the soldiers said. He replied, “Normally, a human king sits inside and his servants stand outside and guard him. But, for the Holy One Blessed be He, His servants are inside and He guards them from outside.” They too converted. Titus sent no more soldiers.