Central theme of Aquinas' theology

"This is eternal life: that they may know Thee, the only true God, and Jesus Christ, whom Thou has sent."

All the knowledge imparted by faith turns about these two points, the divinity of the Trinity and the humanity of Christ. The humanity of Christ is the way by which we come to the divinity. 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.

Light of Faith: The Compendium of Theology (Manchester, New Hampshire: Sophia Institute Press, 1933), p.5.