The death of several martyrs in both Christianity and Judaism for the belief in the One True God and a consequent refusal to bow to other gods who aren’t The Lord of Hosts, the Biblical God, is a definitive proof of the seriousness of the monotheistic doctrine of these religions. God in both the OT and through Christ in the NT made it clear there is none other but He who alone made all things. The Church believes in the Trinity; the Father from whom, the Son, through whom, and the Spirit, in whom all things are. Three really distinct persons, but only One God.
The problem with polytheism is that its illogical. There cannot be more than one beginning, more than one absolute source of all power. If we begin to trace the origin of all things, we cannot go infinitely backward, there has to be a point where it terminates. There must be a point where it does not go further back, there necessarily must be a Being who is Himself uncreated/uncaused, who accounts for the creation of all other things. This is what Christians believe: God is the absolute beginning and end of all things. He himself is uncreated, eternal and infinitely powerful. There cannot be more than one Absolute beginning, more than one Uncaused cause of all things, more than one end of all existence.
"‘You are my witnesses,’ says the Lord, ‘and my servant whom I have chosen, that you may know and believe me and understand that I am he. Before me no god was formed, nor shall there be any after me’" (Is. 43:10).
These are the words with which the Niceno-Constantinopolitan Creed begins. The confession of God's oneness, which has its roots in the divine revelation of the Old Covenant, is inseparable from the profession of God's existence and is equally fundamental. God is unique; there is only one God: "The Christian faith confesses that God is one in nature, substance and essence." To Israel, his chosen, God revealed himself as the only One: "Hear, O Israel: The LORD our God is one LORD; and you shall love the LORD your God with all your heart, and with all your soul, and with all your might."4Through the prophets, God calls Israel and all nations to turn to him, the one and only God: "Turn to me and be saved, all the ends of the earth! For I am God, and there is no other.. . To me every knee shall bow, every tongue shall swear. 'Only in the LORD, it shall be said of me, are righteousness and strength.'" Jesus himself affirms that God is "the one Lord" whom you must love "with all your heart, and with all your soul, and with all your mind, and with all your strength".6 At the same time Jesus gives us to understand that he himself is "the Lord".7 To confess that Jesus is Lord is distinctive of Christian faith. This is not contrary to belief in the One God. Nor does believing in the Holy Spirit as "Lord and giver of life" introduce any division into the One God: We firmly believe and confess without reservation that there is only one true God, eternal infinite (immensus) and unchangeable, incomprehensible, almighty and ineffable, the Father and the Son and the Holy Spirit; three persons indeed, but one essence, substance or nature entirely simple. CCC 200-202
This is what the Fathers of the Church has taught throughout history:
Pope Clement I
“What think you, beloved? Did not Moses know beforehand that this would happen? Undoubtedly he knew; but he acted thus, that there might be no sedition in Israel, and that the name of the true and only God might be glorified; to whom be glory for ever and ever. Amen” (Letter to the Corinthians 43 [A.D. 80]).
Ignatius of Antioch
“There is then one God and Father, and not two or three; one who is, and there is no other besides him, the only true [God]. For ‘the Lord your God,’ says [the Scripture], ‘is one Lord’ [Deut. 6:4]. . . . And there is also one Son, God the Word. . . . And there is also one Paraclete” (Letter to the Philadelphians 2 [A.D. 110]).
“The prophets, who were men of God, lived according to Jesus Christ. For that reason they were persecuted, inspired as they were by his grace to convince the disobedient that there is one God, who manifested himself through his Son, Jesus Christ, who is his Word proceeding from silence, and who was in all respects pleasing to him that sent him” (Letter to the Magnesians 8:1 [A.D. 110]).
“There will be no other God, O Trypho, nor was there from eternity any other existing . . . but he who made and disposed all this universe. Nor do we think that there is one God for us, another for you, but that he alone is God who led your fathers out from Egypt with a strong hand and a high arm. Nor have we trusted in any other, for there is no other, but in him in whom you also have trusted, the God of Abraham, and of Isaac, and of Jacob” (Dialogue With Trypho the Jew 11 [A.D. 155]).
“[God] himself also by his own prophets testifies, when he says, ‘I, God, am the first,’ and after this, ‘And beside me there is no other God’ [Is. 44:6]. On this account, then, as I before said, God did not, when he sent Moses to the Hebrews, mention any name, but by a participle he mystically teaches them that he is the one and only God” (Address to the Greeks 21 [A.D. 170]).
“For the Church, although dispersed throughout the whole world even to the ends of the earth, has received from the apostles and from their disciples the faith in one God, Father Almighty, the Creator of heaven and earth and sea and all that is in them; and in one Jesus Christ, the Son of God” (Against Heresies 1:10:1 [A.D. 189]).
“Nor is he moved by anyone; rather, freely and by his Word he made all things. For he alone is God, he alone is Lord, he alone is Creator, he alone is Father, he alone contains all and commands all to exist” (ibid., 2:1:1).
“Of his own accord and by his own power he made all things and arranged and perfected them; and his will is the substance of all things. He alone, then, is found to be God; he alone is omnipotent, who made all things; he alone is Father, who founded and formed all things, visible and invisible, sensible and insensate, heavenly and earthly, by the Word of his power. And he has fitted and arranged all things by his wisdom; and while he comprehends all, he can be comprehended by none. He is himself the designer, himself the builder, himself the inventor, himself the maker, himself the Lord of all” (ibid., 2:30:9).
“The object of our worship is the one God, who, by the word of his command, by the reason of his plan, and by the strength of his power, has brought forth from nothing for the glory of his majesty this whole construction of elements, bodies, and spirits; whence also the Greeks have bestowed upon the world the name ‘cosmos’” (Apology 17:1 [A.D. 197]).
“There is only one God, and none other besides him, the Creator of the world who brought forth all things out of nothing through his Word, first of all sent forth” (Demurrer Against the Heretics 13:1 [A.D. 200]).
“We do indeed believe that there is only one God, but we believe that under this dispensation, or, as we say, oikonomia, there is also a Son of this one only God, his Word, who proceeded from him and through whom all things were made and without whom nothing was made” (Against Praxeas 2 [A.D. 216]).
The Recognitions of Clement
“[T]hough there are many that are called gods, there is but one true God, according to the testimonies of the scriptures” (Recognitions of Clement 3:75 [A.D. 221]).
The Clementine Homilies
“[T]he Scripture says, ‘As I live, says the Lord, there is no other God but me. I am the first, I am after this; except me there is no god’ [Is. 44:6]. And again: ‘You shall fear the Lord your God, and him only shall you serve’ [Deut. 6:13, Matt. 4:10]. And again: ‘Hear, O Israel, the Lord your God is one Lord’ [Deut. 6:4]. And many passages besides seal with an oath that god is one, and except him there is no god” (Clementine Homilies 16:7 [A.D. 221]).
“The specific points which are clearly handed down through the apostolic preaching are these: First, that there is one God who created and arranged all things, and who, when nothing existed, called all things into existence, and that in the final period this God, just as he had promised beforehand through the prophets, sent the Lord Jesus Christ. Secondly, that Jesus Christ himself, who came, was born of the Father before all creatures; and after he had ministered to the Father in the creation of all things, for through him all things were made” (The Fundamental Doctrines 1:0:4 [A.D. 225]).
“The one God, the first and only, both Creator and Lord of all things, had nothing co-eternal. . . . No, he was one, to himself alone. And when he so willed, he created those things which before had no existence other than in his willing to make them and inasmuch as he had knowledge of what would be, for he also has foreknowledge” (Refutation of All Heresies [A.D. 228]).
“We must therefore believe, according to the rule prescribed, on the Lord, the one true God, and consequently on him whom he has sent, Jesus Christ, who by no means, as we have said, would have linked himself to the Father had he not wished to be understood to be God also, for he would have separated himself from him had he not wished to be understood to be God” (Treatise on the Trinity 16 [A.D. 235]).
“God the Father, founder and Creator of all things, who alone knows no beginning, who is invisible, immeasurable, immortal, and eternal, is one God. Neither his greatness nor his majesty nor his power can possibly be—I should not say exceeded, for they cannot even be equaled. From him . . . the Word was born, his Son. . . . And the latter, since he was born of the Father, is always in the Father. And I indeed say always . . . he that exists before all time must be said to have been in the Father always, for he that exists before all time cannot be spoken of in relation to time. . . . [A]ssuredly, he [the Son] is God, proceeding from God, causing, as Son, a second person after the Father, but not taking away from the Father the fact that God is one” (ibid., 31).
Gregory the Wonderworker
“We therefore acknowledge one true God, the one first cause, and one Son, very God of very God, possessing of nature the Father’s divinity—that is to say, being the same in substance with the Father; and one Holy Spirit, who by nature and in truth sanctifies all, and makes divine, as being of the substance of God. Those who speak either of the Son or of the Holy Spirit as a creature we anathematize” (A Sectional Confession of Faith 15 [A.D. 262]).
“There is one God. . . . There is a perfect Trinity, in glory and eternity and sovereignty, neither divided nor estranged. Wherefore there is nothing either created or in servitude in the Trinity; nor anything superinduced, as if at some former period it was nonexistent, and at some later period it was introduced. And thus neither was the Son ever wanting to the Father, nor the Spirit to the Son; but without variation and without change, the same Trinity abides ever” (Declaration of Faith [A.D. 265]).
Council of Nicaea I
“We believe in one God, the Father Almighty, maker of heaven and earth, of all things, visible and invisible” (Creed of Nicaea [A.D. 325]).
Epiphanius of Salamis
“We believe in one God, the Father Almighty, maker of all things, both visible and invisible; and in one Lord Jesus Christ, the Son of God, begotten of God the Father, only-begotten, that is, of the substance of the Father; God of God, light of light, true God of true God; begotten, not made; consubstantial with the Father; through whom all things were made, both those in heaven and those on earth, both visible and invisible; who for us men and for our salvation came down and took flesh, that is, was born perfectly of the holy ever-virgin Mary by the Holy Spirit, was made man, that is, he received perfect man, soul and body and mind and all that man is, except sin” (The Man Well-Anchored 120 [A.D. 374]).
Patrick of Ireland
“[T]here is no other God, nor has there been heretofore, nor will there be hereafter, except God the Father unbegotten, without beginning, from whom is all beginning, upholding all things, as we say, and his Son Jesus Christ, whom we likewise to confess to have always been with the Father—before the world’s beginning. . . . Jesus Christ is the Lord and God in whom we believe . . . and who has poured out on us abundantly the Holy Spirit . . . whom we confess and adore as one God in the Trinity of the sacred Name” (Confession of St. Patrick 4 [A.D. 452]).
Fulgentius of Ruspe
“True religion consists in the service of the one true God. For it is truth itself that there is one God; and just as, besides the one truth, there is no other truth, so too, besides the one true God there is no other true God. For the one truth itself is naturally one true divinity. And thus one cannot speak truthfully of two true gods, because it is not possible for the truth itself, naturally one, to be divided” (Letters8:10 [A.D. 519]).