All this we know both from the testimonies of Holy Scripture1 and from the respective works of the three Persons, and especially those we perceive in ourselves. The testimonies of Scripture which lead us to
believe this Holy Trinity are written in many places of the Old Testament. It is not necessary to mention them all; it is sufficient to select some with discretion.
In the book of Genesis God says, “Let Us make man in our image after our likeness…So God created man in His own image…; male and female He created them” (Gen 1:26-27). Also: “Behold, the man has become like one of Us” (Gen 3:22). From God’s saying, “Let Us make man in Our image,” it appears that there are more divine persons than one; and when He says, “God created,” He indicates that there is one God. It is true, He does not say how many persons there are, but what seems to be somewhat obscure in the Old Testament is very plain in the New Testament. For when our Lord was baptized in the river Jordan, the voice of the Father was heard, who said, “This is My beloved Son” (Mt 3:17); the Son was seen in the water, and the Holy Spirit descended upon Him in bodily form as a dove.2 For the baptism of all believers Christ prescribed this formula: “Baptize all nations into the Name of the Father, and of the Son, and of the Holy Spirit” (Mt 28:19). In the gospel according to Luke the angel Gabriel thus addressed Mary, the mother of our Lord: “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy, the Son of God” (Lk 1:35). Likewise: “The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all” (2 Cor 13:14). In all these places we are fully taught that there are three persons in one only divine essence.
Although this doctrine far surpasses all human understanding, nevertheless in this life we believe it on the ground of the Word of God, and we expect to enjoy its perfect knowledge and fruit hereafter in heaven.
Moreover, we must observe the distinct offices and works of these three Persons towards us. The Father is called our Creator by His power; the Son is our Saviour and Redeemer by His blood; the Holy Spirit is our Sanctifier by His dwelling in our hearts. The doctrine of the Holy Trinity has always been maintained and preserved in the true church since the time of the apostles to this very day, over against Jews, Muslims, and against false Christians and heretics such as Marcion, Mani, Praxeas, Sabellius, Paul of Samosata, Arius, and such like, who have been justly condemned by the orthodox fathers. In this doctrine, therefore, we willingly receive the three creeds, of the Apostles, of Nicea, and of Athanasius; likewise that which in accordance with them is agreed upon by the early fathers.
1 Jn 14:16, 15:26; Acts 2:32-33; Rom 8:9; Gal 4:6; Tit 3:4-6; 1 Pt 1:2; 1 Jn 4:13-14; 1 Jn 5:1-12; Jude 20-21; Rev 1:4-5