Ask yourselves, how do spellcasters actually bind and control magic? We possess the ability to perceive and touch the Winds of Magic in some way. That we possess this ability does not necessarily imply that we know how to use it, any more than any ordinary Human born with two eyes and two hands can read or write without first being taught. Magisters control magic through the use of techniques and formulae that have been discovered through logical deduction and controlled trial and error across the centuries. On the other hand, priests and clerics are able to perform miracles solely by merit of the strength of their beliefs and faith.
I am sure it has not escaped any of your notice that the more genuinely pious and faithful the cleric, the more successful his or her prayers for divine miracles and blessings seem to be. Have not the Theogonist of Sigmar and even Emperor Magnus himself been known to perform the most startling of feats? I, the elected Supreme Patriarch of the Colleges of Magic, trained by mighty Teclis himself, must stretch myself to replicate any of them, from Magnus's ability to banish the greatest of Daemons, to his recorded ability to dispel sorceries of some of Tzeen'eth's most powerful sorcerers—all because of his genuine and absolute faith in the might of Sigmar Heldenhammer.
Why is it the instinctive beliefs and emotions of clerics are just as successful in controlling magic as the considered, practiced and logically deduced enchantments of the diverse Magisters? Because, my brothers and sisters, although the Aethrian itself has existed since the beginning of time, all momentum, identity, and personality within the Aethrian has been formed by the thoughts and feelings of mortals. That is, the Gods themselves are the creations of mortal experience. Although we mortals are at the mercy of Gods and Daemons, they are still our creations and not the other way around.
If all Gods and Daemons are manifestations of the mortal world's thoughts, dreams, and feelings, then could this not explain why they react so readily to the faith of those who worship them? Could it be that some or all of the Gods actually need the faith of mortals to maintain their unique identities? They could be drawn to particularly strong demonstrations of faith, both to feed off of it and to encourage even greater levels of faith in the believer. This could perhaps explain why the servants of Chaos are often so much stronger in their spellcraft and wield more powerful magics than we Magisters of the Empire. These Sorcerers can both manipulate the Winds of Magic as we Magisters do and also have an immense faith in, and a direct link to, their Daemonic Gods.
Indeed, this could help explain the frightening power of those sorcerers who are dedicated to the Daemon God Tzeentch, for their God is the first and greatest God of the study of magic. In addition to their almost priestly powers of supplication, the Change Lord's Sorcerers also posses a divinely inspired knowledge of spellcraft that often far outstrips that of all other spellcasters.
Even though this might be true, I do not believe that we, the Magisters of the Imperial Colleges of Magic, should follow their example and choose for ourselves Gods to wholly dedicate ourselves to and beg favours from, to use in conjunction with our own arcane spellcraft. Nothing in this life or the next is free, and whilst I am willing to trust in my own abilities and limitations, and accept any errors I make while weaving my spells, I do not wish to trust the continued benevolence of a deity whose need for my faith and dedication might far outweigh my own need for His or Her aid.
—from Magister Volans' third letter to the Colleges of Magic. Extracted from the Liber Chaotica, Compiled by Richter Kless