Maria de Agreda war für den spanischen König Philipp IV. seit 1643, als er ihr zum ersten Mal begegnet war, der wichtigste Freund und Beichtvater in seinem Leben. Ihr vertraute er als einzigem Menschen seine Sorgen, Probleme und Ängste an. Es sind noch heute zahlreiche Briefe zwischen den beiden erhalten geblieben, die uns persönliche Einblicke in die politischen, wirtschaftlichen und privaten Probleme dieses Königs gewähren. Er traf sie durch Zufall, als er sich im Jahr 1643 auf dem Weg nach Aragon befand: "He [Philipp IV.] travelled slowly, nevertheless, and on the 10th July, he approached the Aragonese frontier city of Tarazona, he halted at the humble Convent of the Immaculate Conception at Agreda, which in the previous few years had been founded by a lady whose fame for sanctity and wisdom had already become wide, though she was but forty years of age yet. Maria Coronel had written several mystically religious books, and the convent under her rule was known for its rigidity in an age when most cloisters had grown lax. Philip probably visited the house and its abbess as a usual compliment and duty; but the visit, whatever its motive, set its mark upon him for the rest of his life. The abbess, Sor Maria, as she was called, must have been a woman of worldly wisdom as deep as was her piety. She must have impressed the King, moreover, powerfully as being absolutely disinterested and free from mundane temptation. ... Sor Maria ...seemed to him a very rock of refuge, and in the long talk he had with her she spoke so wisely, yet so fearlessly, ... she urged the King so powerfully to trust to God and himself alone, to work and pray and make his people cleanly, that he went forth from Agreda refreshed in faith and hope, leaving with Sor Maria his command that she was to write to him her private counsel when she listed, and to pray for him and his unceasingly with all her saintly soul. Thenceforward until death snapped the spiritual link that joined them, the heart of Philip was bared in all its sorrow, its weakness, and its sin to Sor Maria alone." (in: Martin Hume: The Court of Philip IV. - Spain in Decadence.,id., pp. 379-380).