ISTANBUL—Pope Francis further demonstrated his commitment to improving relations between Christians and Muslims on Saturday, as he prayed in Istanbul’s historic Blue Mosque and visited the Hagia Sophia—two powerful symbols of the Muslim and Christian faiths. (Further reading: Church must find ways of welcoming divorced Catholics and gays, says Pope Francis).

On the second of a three-day trip to Turkey, the pontiff removed his shoes before entering the Sultan Ahmet Mosque, known as the Blue Mosque for the blue tiles embellishing...