Bartholomew says: "As Innocent III said, there is therefore one true God, the eternal, immense, unchangeable, almighty, and ineffable, Father and Son and Holy Spirit, in fact three persons, but one essence, a substance with an entirely simple nature, the Father from nothing, the Son from the Father, and the Holy Spirit from both without beginning and end, the Father generated, the Son being generated, the Holy Spirit proceeding from the Father and the Son."

Original Latin

Bartholomew's Liber I: (On the Trinity) from De rerum proprietatibus is a short, instructive, credal statement. Its brevity is striking; only Liber X (On the elements: fire, air, earth, and water, but specifically fire), is shorter, but barely. Reviewing and summarizing all theological discussions would have produced much weariness of the flesh and surpassed the expectations and abilities of Bartholomew's intended readers.

Bartholomew is very clear about those whom he addresses, for his selection of material throughout is for the mihi rudi et mei consimilibus , so that the simplices et parvuli, qui propter librorum infinitatem singularum rerum proprietates de quibus tractat scriptura investigare non possunt, in promptu invenire valeant saltem superficialiter quod intendunt.

Bartholomew joined the Franciscan order in its early years, while studying and teaching at the University of Paris. This order, perhaps more rightly known as the Order of Friars Minor (or the Grey Friars, thanks to the color of their habit back then), was established by St. Francis of Assisi with the permission of Pope Innocent III in 1209. Franciscan ideals include humble, Christ-like poverty and service to others, especially preaching, to help individual people of every kind - rich and poor, powerful and powerless - enhance their lives physically and spiritually.

Bartholomew opens with a brief statement of the mystery of the Trinity - three Persons but one essence - and then enters into a discussion of the relationships among the three and how God cannot be perceived directly but only through His effects in the universe. He also discusses the various ways of describing or naming God, so that the learner may come to understand the many aspects of God that produce those effects. For example, there are distinctions among abstract qualities (bonitas , divinitas), concrete or semi-concrete qualities (creator , lumen , sapientia), and descriptive adjectives (justus, generans), as opposed to nouns.

Descriptions of the essence of God can be more philosophically difficult but mystically appealing. Bartholomew repeats one that philosophers from the Middle Ages often ascribed to an ancient mage, Hermes Termegistus: Deus est sphera intellectualis cuius centrum ubique est, circumferentia vero nusquam. Considering his readers' rudimentary learning, he explains how this may be understood: Divina enim essentia in se considerata perfecta est ad modum sphaere, quia non habet principium neque finem; sed prout consideratur ut causa deducens res in esse et eas limitans et finiens, sic dicitur centrum, quia sicut centrum finit lineas et ab ipso lineae deducuntur, ita Deus deducit creaturas et limitat et finit eas.

Bartholomew also includes lists or brief discussions of proper nouns used to name the three persons of the Trinity, such as El, Sabaoth, Tetragrammaton, and Eloym for God the Father; Christus, Salvator, and Verbum caro factum for God the Son; and Columba, Ignis, and Oleum for God the Holy Spirit. And he makes a point of noting how aspects of God can sometimes be named metaphorically because physical features cannot be applied to God. Indeed, sometimes he uses metaphors simply to draw our attention to a point: thus, Os et labia et fauces habere dicitur quia illa velut loquendo per inspirationem nobis revelat, que hominum cogitatio per huiusmodi organa exprimit et ostendit.