Who Said That?

Can anyone, without Googling, identify the source of this quotation?

"That the Roman Church is more honored by God than all others is not to be doubted. St, Peter and St. Paul, forty-six Popes, some hundreds of thousands of martyrs, have laid down their lives in its communion, having overcome Hell and the world; so that the eyes of God rest on the Roman church with special favor. Though nowadays everything is in a wretched state, it is no ground for separating from the Church. On the contrary, the worse things are going, the more should we hold close to her, for it is not by separating from the Church that we can make her better. We must not separate from God on account of any work of the devil, nor cease to have fellowship with the children of God who are still abiding in the pale of Rome on account of the multitude of the ungodly. There is no sin, no amount of evil, which should be permitted to dissolve the bond of charity or break the bond of unity of the body. For love can do all things, and nothing is difficult to those who are united."

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***UPDATE***

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Surprise, surprise: It was from a letter of Martin Luther to Pope Leo. How's that for irony?