To explain this page, and what Merlin does, best is a quote from the introduction to the 9th century Arabic occult magic book Picatrix:
“In the magic of Picatrix, the sources of magical power are in the macrocosm rather than the microcosm; power is native to the universe, not to the mage. Vast currents of creative force set in motion by the Godhead itself cast downward through multiple levels of being, they are refracted by the stars and planets like rays of light striking moving crystals, and descend to the earth with greater or lesser force, depending on the complex geometries of astrological relationship. The magician is the one who knows how to catch these currents at the moments of their greatest power, store their energies in material objects appropriate to them, and direct those energies to carefully chosen ends.” [from the Illustrated Picatrix, translated and annotated by John Michael Greer and Christopher Warnock]
Merlin, then, as a mage, is not a source of power. He just knows how to read the heavens and petition the powers above to help him do what is requested of him. (To do this, he’s got an intense and thorough knowledge of astrology and the magical properties of endless ingredients.) Shoeing this into Christianity is what gives us the idea presented in the second panel, showing the Virgin Mary and the Christ child in control of some celestial orb, surrounded by the zodiac. God put the stars and the planets in the sky, and therefore astrology is some tool to be used to help his will appear on earth. That is the “will of heaven” Merlin speaks of here.
This second panel on page 116 paraphrases the Picatrix directly: “The spirit of the magician furthers the effects of the heavens just as the harvests of nature—that is, in the way that harvests are furthered by plowing and cultivating the earth.” You can assume, then, that Merlin has (a very precious and surely inordinately expensive) copy of this book in his library and is extremely familiar with it, since his inner narrative to us harkens to it.
The zodiac figures here were inspired by those that appear in the Très Riches Heures du Duc de Berry.