God cannot be defined, for the reason that He is immense, and because we are ignorant of His essence. We may, however, describe Him to a certain extent from the revelation which He has been pleased to make of Himself; yet in giving a description of God we must be careful to include in it those attributes, representations and peculiar works, which distinguish Him from all false deities.
A theological and more complete description of God, the one which the church receives, is the following: God is a spiritual essence, intelligent, eternal, different from all creatures, incomprehensible, most perfect in Himself, immutable, of immense power, wisdom and goodness; just, true, pure, merciful, bountiful, most free, hating sin — which is, the eternal Father, who from eternity begat the Son in His own image; the Son, who is the co-eternal image of the Father; and the Holy Spirit, proceeding from the Father and the Son, as has been divinely revealed by the sure word delivered by the Prophets and Apostles, and divine testimonies; that the eternal Father, with the Son and the Holy Spirit did create heaven and earth, and all creatures, is present with all creatures, that He may preserve and rule them by His providence, and produce all good things in them; and that from the human race, made after His own image, He has chosen and gathers unto Himself an everlasting church, by and for the sake of His Son, that by the church this one and true Deity may, according to the word revealed from heaven, be here known and praised, and glorified in the life to come; and that He is the judge of the righteous and the wicked.
This theological description of God, which the church gives, differs from the philosophical description, 1. In perfection, because it contains certain things unknown to men by nature, such as the distinction which exists between the persons of the Godhead, election, and the gathering of the church through the Son. It also explains more fully those things which are known from nature. 2. In its effect, inasmuch as men cannot by the mere light of nature arrive at a true knowledge of God, nor be excited thereby to holiness or to the love and fear of God.
Zacharias Ursinus, Commentary On The Heidelberg Catechism, pp. 123, 124