3. In the unity of the Godhead there be three persons, of one substance, power, and eternity: God the Father, God the Son, and God the Holy Ghost: the Father is of none, neither begotten, nor proceeding; the Son is eternally begotten of the Father; the Holy Ghost eternally proceeding from the Father and the Son.
WLC Q&A 8-11
Q. 8. Are there more Gods than one?
A. There is but one only, the living and true God.
Q. 9. How many persons are there in the Godhead?
A. There be three persons in the Godhead, the Father, the Son, and the Holy Ghost; and these three are one true, eternal God, the same in substance, equal in power and glory; although distinguished by their personal properties.
Q. 10. What are the personal properties of the three persons in the Godhead?
A. It is proper to the Father to beget the Son, and to the Son to be begotten of the Father, and to the Holy Ghost to proceed from the Father and the Son from all eternity.
Q. 11. How doth it appear that the Son and the Holy Ghost are God equal with the Father?
A. The Scriptures manifest that the Son and the Holy Ghost are God equal with the Father, ascribing unto them such names, attributes, works, and worship, as are proper to God only.
WSC Q&A 5-6
Q. 5. Are there more Gods than one?
A. There is but one only, the living and true God.
Q. 6. How many persons are there in the godhead?
A. There are three persons in the Godhead; the Father, the Son, and the Holy Ghost; and these three are one God, the same in substance, equal in power and glory.
HC Q&A 24-25
24. HOW ARE THESE ARTICLES DIVIDED?
Into three parts: the first is of God the Father and our creation; the second, of God the Son and our redemption; the third, of God the Holy Spirit and our sanctification.
 1 Pt 1:2
25. SINCE THERE IS BUT ONE DIVINE BEING, WHY DO YOU SPEAK OF THREE PERSONS: FATHER, SON, AND HOLY SPIRIT?
Because God has so revealed Himself in His Word, that these three distinct persons are the one, true, eternal God.
Belgic Confession Article 8-9
ARTICLE 8 – GOD IS ONE IN ESSENCE, YET DISTINGUISHED IN THREE PERSONS
According to this truth and this Word of God, we believe in one only God,1 who is one single essence, in which are three persons, really, truly, and eternally distinct according to their incommunicable properties; namely, the Father, the Son, and the Holy Spirit.2 The Father is the cause, origin, and beginning of all things visible and invisible.3 The Son is the Word, the wisdom, and the image of the Father.4 The Holy Spirit is the eternal power and might who proceeds from the Father and the Son.5 Nevertheless, God is not by this distinction divided into three, since the Holy Scriptures teach us that the Father, the Son, and the Holy Spirit each has His personal existence, distinguished by Their properties; but in such a way that these three persons are but one only God.
It is therefore evident that the Father is not the Son, nor the Son the Father, and likewise the Holy Spirit is neither the Father nor the Son. Nevertheless, these persons thus distinguished are not divided, nor intermixed; for the Father has not assumed our flesh and blood, neither has the Holy Spirit, but the Son only. The Father has never been without His Son,6 or without His Holy Spirit. For these three, in one and the same essence, are equal in eternity. There is neither first nor last; for They are all three one, in truth, in power, in goodness, and in mercy.
ARTICLE 9 – SCRIPTURE PROOF OF THIS DOCTRINE
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 (Mat 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 (Mat 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 (Luke 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.
The Athanasian Creed
Whoever desires to be saved should above all hold to the catholic faith.
Anyone who does not keep it whole and unbroken will doubtless perish eternally.
Now this is the catholic faith:
That we worship one God in trinity and the trinity in unity,
neither blending their persons
nor dividing their essence.
For the person of the Father is a distinct person,
the person of the Son is another,
and that of the Holy Spirit still another.
But the divinity of the Father, Son, and Holy Spirit is one,
their glory equal, their majesty coeternal.
What quality the Father has, the Son has, and the Holy Spirit has.
The Father is uncreated,
the Son is uncreated,
the Holy Spirit is uncreated.
The Father is immeasurable,
the Son is immeasurable,
the Holy Spirit is immeasurable.
The Father is eternal,
the Son is eternal,
the Holy Spirit is eternal.
And yet there are not three eternal beings;
there is but one eternal being.
So too there are not three uncreated or immeasurable beings;
there is but one uncreated and immeasurable being.
Similarly, the Father is almighty,
the Son is almighty,
the Holy Spirit is almighty.
Yet there are not three almighty beings;
there is but one almighty being.
Thus the Father is God,
the Son is God,
the Holy Spirit is God.
Yet there are not three gods;
there is but one God.
Thus the Father is Lord,
the Son is Lord,
the Holy Spirit is Lord.
Yet there are not three lords;
there is but one Lord.
Just as Christian truth compels us
to confess each person individually
as both God and Lord,
so catholic religion forbids us
to say that there are three gods or lords.
The Father was neither made nor created nor begotten from anyone.
The Son was neither made nor created;
he was begotten from the Father alone.
The Holy Spirit was neither made nor created nor begotten;
he proceeds from the Father and the Son.
Accordingly there is one Father, not three fathers;
there is one Son, not three sons;
there is one Holy Spirit, not three holy spirits.
Nothing in this trinity is before or after,
nothing is greater or smaller;
in their entirety the three persons
are coeternal and coequal with each other.
So in everything, as was said earlier,
we must worship their trinity in their unity
and their unity in their trinity.
Anyone then who desires to be saved
should think thus about the trinity.