'The Throne of Heaven' refers to a Heavenly Construct that grants the appeals and wishes of prayers done by mortal men. In the context of Miracles, it is a system by which the reality of the Mortal World is altered through the power of a Divine Construct. According to teachings, the Throne of Heaven is continually monitoring humanity, and is said to be responsible for blessings or particularly good luck. Though people most often appeal to the Throne of Heaven by communicating their appeals through prayer when they are faced with difficulty. Requests to the Throne of Heaven can be amplified by invoking the name of an Angel or Saint who can directly communicate with the Throne of Heaven. Also having knowledge of the 'Antioch Format' - principles of Language and Format for prayers recognized at the Council of Nicaea - also further amplifies the clarity of the request. Fundamentally, a Prayer is a Human's Spirit Appealing to the Throne of Heaven to change reality in the Mortal Plain.
Hospitaller's own soul is a Divine Construct itself. During the Crusades, the Papacy of Rome promised salvation to those who took up arms to Crusade in the Holy Land. A portion of those souls would be involved with the Knights Hospitaller. The souls of those people involved with the are the basis of what made Hospitaller's own soul. This soul, being made up of so many entities that now reside in Heaven, are able to directly appeal to the Throne of Heaven. Meaning Hospitaller's own prayers, possessing many direct conduits to the Throne of Heaven, can potentially invoke reality changes on the level of 'Acts of God' in the Mortal Plain.
For example, in the Rite of Exorcism - a prayer utilized to cast out demons. A trained clergymen is typically required in order to see it done correctly. Since the right of exorcism is a prayer, it is also an appeal to the Throne of Heaven. Hospitaller's effectiveness in invoking this particular prayer should be magnitudes beyond what a clergyman can do. To speak it colorfully, having a 'True Hospitaller' invoke the right of exorcism to get rid of one spirit would be like slamming an asteroid into a planet in order to deal with a single cockroach.
Mercifully, This Hopitaller isn't aware of this particular ability. The current Hospitaller is a fragment of a true angel bound inside a physical body. She isn't quite aware of her connection with the Throne of Heaven at the start of the story, thus her relay efficiency is sorely cut down. Overall, Hospitaller's circumstances make her very 'watered down', but she would still be something to be genuinely concerned about if a targeted character saw the whole picture behind the girl with the tiara bolted on her head.