The below was taken from here. I will beginning discussing this shortly.
On the Errors of the Trinity
Michael Serveto, alias Reves
A Spaniard of Aragon
Due to the size of Servetus’ book I will provide his arguments (as given by the translator) for each chapter (book). Below, you will find the links to Books VI and VII. Maybe? at some future date, I may add more.
Any discussion of the Trinity should start with the man. That Jesus, surnamed Christ, was not a Hypostasis but a human being is taught both by the early Fathers and in Scriptures, taken in their literal sense, and is indicated by the miracles he wrought. He, and not the Word, is also the miraculously born Son of God in fleshly form, as the Scriptures teach – not a hypostasis, but an actual Son. He is God, sharing God’s divinity in full; and the theory of a communicatio idiomatum is a confusing sophistical quibble. This does not imply two Gods, but only a double use of the term God, as is clear from the Hebrew use of the term. Christ being one with God the Father, equal in power, came down from heaven and assumed flesh as a man. In short, all the Scriptures speak of Christ as a man.
The doctrine of the Holy Spirit as a third separate being lands us in practical tritheism, even though the unity of God be insisted on. Careful interpretation of the usual proof-text shows that they teach not a unison of three beings in one, but a harmony between them. The Holy Spirit as a third person of the Godhead is unknown in Scripture. It is not a separate being, but an activity of God himself. The doctrine of the Trinity can be neither established by logic nor proved from Scriptures, and is in fact inconceivable. There are many reasons against it. The Scriptures and the Fathers teach one God the Father, and Jesus Christ his Son: but scholastic philosophy has introduced terms which are not understood, and do not accord with Scripture. Jesus taught that he himself was the Son of God. Numerous heresies have sprung from this philosophy, and fruitless questions have arisen out of it. Worst of all, the doctrine of the Trinity incurs the ridicule of the Mohammedans and the Jews. It arose out of Greek philosophy rather than from the belief that Jesus Christ is the Son of God; and he will be with the Church only if it keeps his teaching.
CHRIST, the Son of man, who descended from heaven, was the Word by uttering which God created the world. He became flesh as God’s firstborn, and was the Son of God. He was both human and divine. God’s Spirit, moving all things, operates within us as the Holy Spirit, which is a person of the Godhead. It proceeds from the Son, not as a separate being but as a ministering spirit. It is holy, one of three persons in the Godhead, and sanctifies us by dwelling within us.
The pre-existent Word, first uttered by God in creation, was afterwards incarnate in Jesus as the Son of God. Christ spirit manifested the power of God’s Word in creation and in the world, and he derives our holy service; yet the Father did not suffer in Christ body. High praise is ascribed to Christ as the wisdom of God. The Word was not the Son, but a disposition of God, who is above all distinctions of time. Belief that Jesus is the Christ, the Son of God, is the essence of Christian faith, and the foundation of the Church.
God has manifest himself in three different dispositions. Of these, the Holy Spirit is his activity in the spirit of man, and is the minister of the Word. God is seen in the Person of Christ, represented in Scripture the imagery of angels; but the real image of God is Christ. The term Nature is appropriate only to God; the Word no longer exist; Person means a representation of another being; Christ incarnate, is the image of the Substance, but not of the Nature, of God.
Examination of the Old Testament usage of the words for God – Elohim and Jehovah – shows that both refer to Christ, as centre of all, and the essence of all things.
The incomprehensible God is known through Christ, by faith, rather than by philosophical speculation. He manifests God to us, being the expression of his very being; and through him alone God can be known. The Scriptures reveal him to those who have faith; and thus we come to know the Holy Spirit as the divine impulse working in us.
The eternally begotten Son was a spoken word by which God made himself known. The Hebrew shows that the whole nature of God abode in Christ as Elohim, man being blended with God. The Word was a disposition of God, who begot the Son, a visible being. The Holy Spirit also is a real being as Christ was. The Word was an actual being, creating all things, manifesting God in bodily form.