Thomas Aquinas, Compendium of Theology

Faith is a certain foretaste of that knowledge which is to make us happy in the life to come. The Apostle says, in Hebrews 11:1, that faith is "the substance of things to be hoped for," as though implying that faith is already, in some preliminary way, inaugurating in us the things that are to be hoped for, that is, future beatitude. Our Lord has taught us that this beatific knowledge has to do with two truths, namely, the divinity of the Blessed Trinity and the humanity of Christ. That is why, addressing the Father, He says: "This is eternal life: that they may know Thee, the only true God, and Jesus Christ, whom Thou hast sent" (John 17:3). All the knowledge imparted by faith turns about these two points, the divinity of the Trinity and the humanity of Christ. This should cause us no surprise: the humanity of Christ is the way by which we come to the divinity. Therefore, while we are still wayfarers, we ought to know the road leading to our goal. In the heavenly fatherland adequate thanks would not be rendered to God if men had no knowledge of the way by which they are saved. This is the meaning of our Lord's words to His disciples: "And whither I go you know, and the way you know" (John 14:4).

Three truths must be known about the divinity: first the unity of the divine essence, secondly the Trinity of persons, and thirdly the effects wrought by the divinity.