Since there is but one only divine essence, (a) why speakest thou of Father, Son, and Holy Ghost?

Because God has so revealed himself in his word, (b) that these three distinct persons are the one only true and eternal God.

(a) Deut.6:4 Hear, O Israel: The LORD our God is one LORD: Eph.4:6 One God and Father of all, who is above all, and through all, and in you all. Isa.44:6 Thus saith the LORD the King of Israel, and his redeemer the LORD of hosts; I am the first, and I am the last; and beside me there is no God. Isa.45:5 I am the LORD, and there is none else, there is no God beside me: I girded thee, though thou hast not known me: 1 Cor.8:4 As concerning therefore the eating of those things that are offered in sacrifice unto idols, we know that an idol is nothing in the world, and that there is none other God but one. 1 Cor.8:6 But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him. (b) Isa.61:1 The Spirit of the Lord GOD is upon me; because the LORD hath anointed me to preach good tidings unto the meek; he hath sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to them that are bound; Luke 4:18 The Spirit of the Lord is upon me, because he hath anointed me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach deliverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised, Gen.1:2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters. Gen.1:3 And God said, Let there be light: and there was light. Ps.33:6 By the word of the LORD were the heavens made; and all the host of them by the breath of his mouth. Isa.48:16 Come ye near unto me, hear ye this; I have not spoken in secret from the beginning; from the time that it was, there am I: and now the Lord GOD, and his Spirit, hath sent me. Ps.110:1 <> The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool. Matt.3:16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: Matt.3:17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased. Matt.28:19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: 1 John 5:7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. Isa.6:1 In the year that king Uzziah died I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple. Isa.6:3 And one cried unto another, and said, Holy, holy, holy, is the LORD of hosts: the whole earth is full of his glory. John 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you. John 15:26 But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me: 2 Cor.13:13 All the saints salute you. Gal.4:6 And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father. Eph.2:18 For through him we both have access by one Spirit unto the Father. Tit.3:5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost; Tit.3:6 Which he shed on us abundantly through Jesus Christ our Saviour;