All this we know as well from the testimonies of Holy Writ as from their operations, and chiefly by those we feel in ourselves. The testimonies of the Holy Scriptures that teach us to believe this Holy Trinity are written in many places of the Old Testament, which are not so necessary to enumerate as to choose them out with discretion and judgment.
In Genesis, chap. 1:26, 27, God says: Let us make man in our image, after our likeness, etc. And God created man in his own image, male and female created he them. And Gen. 3:22, Behold, the man is become as one of us. From this saying, Let us make man in our image, it appears that there are more persons than one in the Godhead; and when He says, God created, He signifies the unity. It is true, He does not say how many persons there are, but that which appears to us somewhat obscure in the Old Testament is very plain in the New. For when our Lord was baptized in Jordan, the voice of the Father was heard, saying, This is my beloved Son; the Son was seen in the water, and the Holy Spirit appeared in the shape of a dove. This form is also instituted by Christ in the baptism of all believers: Make disciples of all the nations, baptizing them into the name of the Father and of the Son and of the Holy Spirit. In the Gospel of Luke the angel Gabriel thus addressed Mary, the mother of our Lord: The Holy Spirit shall come upon thee, and the power of the Most High shall overshadow thee; wherefore also the holy thing which is begotten shall be called the Son of God. Likewise: The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Spirit, be with you all. And (A.V.): There are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
In all these places we are fully taught that there are three persons in one only divine essence. And although this doctrine far surpasses all human understanding, nevertheless we now believe it by means of the Word of God, but expect hereafter to enjoy the perfect knowledge and benefit thereof in heaven.
Moreover, we must observe the particular offices and operations of these three persons towards us. The Father is called our Creator, by His power; the Son is our Savior and Redeemer, by His blood; the Holy Spirit is our Sanctifier, by His dwelling in our hearts.This doctrine of the Holy Trinity has always been affirmed and maintained by the true Church since the time of the apostles to this very day against the Jews, Mohammedans, and some false Christians and heretics, as Marcion, Manes, Praxeas, Sabellius, Samosatenus, Arius, and such like, who have been justly condemned by the orthodox fathers. Therefore, in this point, we do willingly receive the three creeds, namely, that of the Apostles, of Nicea, and of Athanasius; likewise that which, conformable thereunto, is agreed upon by the ancient fathers.