Here is the quote in context. Another poster has provided a link for the entire speech which was, IMHO, excellent. The quote is in blue…
Without descending to details, such as the difference in treatment accorded to those who have the “Book” and the “infidels,” he turns to his interlocutor somewhat brusquely with the central question on the relationship between religion and violence in general, in these words: “Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached.”
The emperor goes on to explain in detail the reasons why spreading the faith through violence is something unreasonable. Violence is incompatible with the nature of God and the nature of the soul. “God is not pleased by blood, and not acting reasonably (“syn logo”) is contrary to God’s nature. Faith is born of the soul, not the body. Whoever would lead someone to faith needs the ability to speak well and to reason properly, without violence and threats… To convince a reasonable soul, one does not need a strong arm, or weapons of any kind, or any other means of threatening a person with death…”
The whole point of the pope’s speech is that God is LOGOS which means both Word and Reason. Therefore He is Reason and will not act un-reasonably. Violence is unreasonable. Therefore, compelling conversion through violence is against the very nature of God as it is unreasonable.
In context, the pope’s reference makes perfect sense.
Please, take time to read the speech in its entirety.