This same description teaches that the true God, whom the church worships, may be distinguished from false gods in three ways: by His attributes, personal distinctions, and works. God has declared by His works that He is such an one by nature as His attributes import. He also shows that there are three persons in one divine essence, since, according to His works, which are works either of creation, or of redemption, or sanctification, God has different titles attributed to Him, and to each person of the Godhead there is a peculiar name applied. God, therefore, differs from idols.
First, by His attributes. Out of the church no attributes of God can be rightly and fully known. Even His mercy is not properly known by those who are out of the church, because the Son is not known, or the doctrine concerning Him is corrupted. Nor do they know His justice, because the wicked do not believe that God is so greatly offended at sin that any satisfaction was needed, or that redemption could be effected only by the death of His Son. Nor can the wisdom of God be known without the church, because the principal part of it is found in His word, which the Gentiles had not.
The same thing may be said of the truth of God, because we do not gain a knowledge of His promises from nature; and so of all the divine attributes. The church, however, attributes to God, in the highest degree, righteousness, truth, goodness, mercy, loving kindness; which attributes of God the various sects are either entirely ignorant of, or, if they have any knowledge of them, they misrepresent them.
Secondly, by the personal distinctions of the Godhead. The heathen philosophers and secularists neither know nor acknowledge that there are three persons in one divine essence. The church, however, acknowledges and calls upon the Father, Son, and Holy Spirit, one God, subsisting in three persons, as He has revealed Himself in His word.
Thirdly, by His works. Those who are without the church have no proper knowledge of the creation and government of all things, much less have they a correct knowledge of the work of redemption and sanctification through the Son and the Holy Spirit. The true God is, in these respects, distinguished from idols. The knowledge of God, which His word reveals to the church, is also different from that which the heathen have obtained from the light of nature.
Zacharias Ursinus, Commentary On The Heidelberg Catechism, pp. 124, 125