Question: Now that we know the benefits we receive from the true knowledge and confession of God as well as the danger if we depart from it, let us go on in the doctrine of the true knowledge of God. Tell me, since there is but one divine Being, why do you speak of three: Father, Son, and Holy Spirit?
In order that we might know and regard God in the way that He has revealed Himself in His Word and made Himself known. God has made Himself known in no other way than that these three distinct persons are one, true, eternal God.
Question: Show from Scripture that the one divine Being has revealed Himself as three distinct persons in this one Being.
Deuteronomy 6:4 “Hear, O Israel: The Lord our God is one God.” Present at creation are the Father, the Word (which is the Son, John 1:1-3), and the Holy Spirit (Genesis 1:2). Likewise, in Mark 1:10, 11 the Father bears witness from heaven to His Son, and the Holy Spirit descends upon Him in the form of a dove. Or again, Matthew 28:19: “Go forth and teach all nations, and baptize them in the name of the Father, Son, and Holy Spirit.” Now we are never baptized in the name of a creature, that is, in order to believe in a creature (1 Corinthians 1:13). Thus we should be certain from our baptism that these three persons are the one true God.
12 Now I mean this, that each one of you is saying, “I am of Paul,” and “I of Apollos,” and “I of Cephas,” and “I of Christ.” 13 Has Christ been divided? Paul was not crucified for you, was he? Or were you baptized in the name of Paul?
Caspar Olevianus, A Firm Foundation, p. 18