The famous but enigmatic definition of God as “an infinite sphere whose center is everywhere and circumference nowhere” comes from the Book of Twenty-Four Philosophers, attested in the 12th century. You can find the text and explanation here (“Rule II”), though the explanation doesn’t shed as much light as you might like on what the definition means.
Alan of Lille, another 12th-century author, gives this explanation of the definition (with one change in wording), which I translate here in case anyone on the Internet would find it helpful (this is from his Theologicae Regulae, Rule VII, text from Migne, p. 627).
God is an intelligible sphere, whose center is everywhere and circumference nowhere. The rule above (“Only the Monad is Alpha and Omega”) proves this. For insofar as God lacks a beginning and end, he is referred to as a sphere; for it is a property of the spherical shape that it lack a beginning and end.
But he is not a bodily sphere, but an intelligible sphere. For when we call God a sphere, we ought not go down the path of imagination, so as to imagine him as a sphere in the likeness of physical bodies. But, led by the intelligence, by that definition we understand God himself to be a sphere because he is eternal. Thus, according to Martianus Capella, a crown [diadema] refers to possessing eternity, since it signifies the lack of a beginning or end. Thus the lack of a beginning and end is called a crown [diadema]- a “double-lack” [duo demens], as it were.
Then we have: …whose center is everywhere and circumference nowhere. O great difference between bodily and intelligible spheres! The center of a bodily sphere can hardly be said to be anywhere at all, it’s so miniscule, while its circumference is understood to occupy several points. But the center of the Intelligible Sohere is everywhere, and its circumference nowhere. “Center” refers to creation, for just as Time is considered but a moment in comparison with Eternity, so the creature compared to the immensity of God is but a point or center. Therefore God’s immensity is referred to as his circumference, since by ordering all things, he in a certain sense surrounds all things, and everything is embraced under his immensity.
There’s also this other difference between the bodily and intelligible spheres: bodily spheres have an immobile center and mobile circumference. But it’s the opposite for the intelligible sphere: God, remaining stable, gives motion to everything else.
 cf. Bonaventure, Itinerarium 6.2.
 As in the heavenly spheres, the orbits of the planets.
Deus est spaera intelligibilis, cujus centrum ubique, circumferentia nusquam. Hanc probat illa regula, qua dictum est, solam monadem esse alpha et omega: ex eo enim quod principio caret, et fine Deus, spaera dicitur: proprium enim spaericae formae est principio et fine carere.
Sed non est spaera corporalis, imo intelligibilis. Cum enim Deum spaeram esse dicimus, non oportet nos deduci ad imaginationes, ut imaginemur eum esse spaeram ad similitudinem corporum; sed, duce intelligentia, ea ratione intelligamus ipsum Deum esse spaeram quia aeternus est. Unde apud Martianum diadema dicitur aeternitas habere, quia principio et fine carere significatur. Unde carentia principii et finis diadema, quasi duo demens, id est principium et finem, apellatur.
Sequitur: cujus centrum ubique, circumferentia nusquam. O magna inter spaeram corporalem, et intelligibilem differentia! In spaera corporali centrum propter sui parvitatem vix alicubi esse perpenditur, circumferentia vero in pluribus locis esse comprehenditur. In intelligibili vero spaera centrum ubique, circumferentia nusquam. Centrum dicitur craetura, quia sicut tempus collatum aeternitati reputatur momentum, sic creatura immensitati Dei comparata, punctum, vel centrum. Immensitas ergo Dei circumferentia dicitur, quia omnia disponendo quodam modo omnipus circumfertur, et omnia infra suam immensitatem complectitur.
Haec etiam alia differentia inter spaeram corporalem, et intelligibilem, quia spaerae corporales centrum immobile, circumferentia mobilis; in spaera intelligibili contra, quia Deus stabiles manens dat cuncta moveri.