"I am the Lord, and there is no other; Besides Me there is no God," (Isaiah 45:5).
Historically, one of the biggest questions that Christians had to address was why they limit themselves to only one God. Many Christians in the modern West have never faced this question, instead typically having to defend why they believe in any God at all. The ancient question that Christians in ages past had to answer from their pagan neighbors has not disappeared, however, and logically it is not enough to say that ancient polytheists were wrong just because no one believes that anymore. Even if that were enough, it's not actually true to say that no one believes it. Outside of our bubble of the secular West, the world is filled with local, folk religions that worship myriads of deities. Not only that, Hinduism is one of the world's largest religions, and if we take all forms of Hinduism together, the religion contains literally millions of gods and goddesses. Even the modern, American religion of Mormonism teaches that there are innumerable gods over countless worlds. Indeed, the goal of faithful Mormons is to eventually become gods themselves and rule over their own worlds one day. We still live in a world that believes in many gods, and yet the Christian's confession ever remains that there is one and only one God, the sovereign Creator of all that exists. Why should anyone believe this claim to be true?
The Biblical Testimony
First and foremost, the Christian knows that there is only one God because God has made Himself known in Scripture and made it clear through His mighty works of old and through the words of the prophets that He alone is the true and living God. Through Moses, God told the people of Israel:
"To you it was shown that you might know that the Lord, He is God; there is no other besides Him," (Deuteronomy 4:35).
"Know therefore today, and take it to your heart, that the Lord, He is God in heaven above and on the earth below; there is no other," (Deuteronomy 4:39).
Shortly afterward, Moses spoke the words that Jesus would later affirm as the greatest of all the commandments:
"Hear, O Israel! The Lord is our God, the Lord is one! You shall love the Lord your God with all your heart and with all your soul and with all your might," (Deuteronomy 6:4-5).
God also said through the Prophet Joel:
"Thus you will know that I am in the midst of Israel, And that I am the Lord your God, And there is no other; And My people will never be put to shame," (Joel 2:27).
And through the Prophet Isaiah He likewise echoed:
"Declare and set forth your case; Indeed, let them consult together. Who has announced this from of old? Who has long since declared it? Is it not I, the Lord? And there is no other God besides Me, A righteous God and a Savior; There is none except Me," (Isaiah 45:21).
"Remember the former things long past, For I am God, and there is no other; I am God, and there is no one like Me," (Isaiah 46:9).
And in case someone might try to misconstrue these plain words as only saying that there is one God for Israel but leaving open the possibility of other gods for other peoples, the Bible is clear:
"Turn to Me and be saved, all the ends of the earth; For I am God, and there is no other," (Isaiah 45:22).
And when Naaman, the captain of the army of Aram, came to the prophet Elisha and was healed of an incurable skin disease, he confessed:
"Behold, now I know that there is no God in all the earth but in Israel," (2 Kings 5:15).
The testimony of God Himself in the words of Scripture is thus clear: there is only one God, and we should worship Him alone.
By definition, there is only one God
When a Christians speak of God, we don't mean just any spiritual being. The biblical worldview includes angels and demons, so the idea of many powerful spirits in the world is not in dispute. The question is: are those spirits actually gods? Are they true deities and proper objects of human worship? The answer is that is no; they are not.
When we speak of God, we are talking about the Supreme Being. God is ultimate and eternal. God is, as a Christian thinker named Anselm of Canterbury put it, "that than which no greater can be conceived." There cannot be two Supreme Beings for the obvious reason that neither of them would be supreme. There cannot be several ultimate realities because none of them would be ultimate. It is impossible for there to be multiple "greatest conceivable beings," both because "greatest" must mean greater than all others, but also because I can conceive of a being that is great beyond any possible rivals. If there are many of something, I can easily conceive of something greater than that.
God is also eternal, all powerful, all knowing, ever present, and wholly good. But what would it mean for there to be two beings that are eternal, all powerful, all knowing, ever present, and wholly good? They cannot be distinguished by time, space, substance, knowledge, ability, or desire. What divides them? What makes them two distinct beings rather than one? I can speak of two different human beings because they have two separate bodies in two distinct places. They have different desires and moral inclinations. They are born at different times and they die at different times. They are distinct in location, life, and learning. But that cannot be said of two uncreated, omnipotent, omniscient, omnipresent, morally perfect Gods. God can be (indeed, He must be) distinct from creation, for God is eternal, and creation is temporal; God is infinite, and creation is finite; God's knowledge and power are unlimited, and every created thing has clear limits. But it would be impossible to distinguish two infinite and unlimited Gods. The concept of two such Gods is completely incoherent. There is, and can only be, one true God.
The only way around this is to lower the bar for what it means to be a god, which is generally what polytheists do. The "gods" are often born or formed at various times and might be limited to particular places. The "gods" have power over certain things and sometimes even need things from us. Men form idols with their own hands and must guard them against theft and shield them from the elements because the "gods" cannot protect their own sacred images. I have walked through temples in Asia where the "gods" are locked in cages at night for their own safety. But, by definition, gods that have beginnings, are bound by physical locations, or whose power is limited by some outside constraint owe their existence (and thus their allegiance and submission) to something greater than themselves. I ought not to worship such finite spirits; even if I believe them to exist. Instead, I ought to give my devotion and adoration to the one and only God who made both them and me. The uncreated source of all created things who does not owe His existence to anyone or anything else, this is the one whom I ought to worship. Just as it would be an offense to give other men the respect due to my own father or to give other women the intimacy due only to my own wife, how much more evil is it for me to give unworthy spirits the honor and worship due only to the Supreme and Eternal God?
It is also noteworthy that virtually all of the logical arguments that demonstrate that God exists also point unanimously to a single God. Whether one is dealing with the Cosmological Argument, the Transcendental Argument, the Ontological Argument, the Moral Argument, or most any of the others, the logical conclusion is not a pantheon of finite deities but rather one supreme, uncaused maker and sustainer of all things.
If we are to believe in, honor, worship, and submit to the one and only God, it is not enough to have a vague notion that there is some higher power out there, even if we concede that there is only one such higher power. We must seek to know who this God is. As noted above, the Bible presents us with this one true God in a specific and detailed self-revelation. When Moses asked God to make Himself known and to reveal His name, God replied, "I AM THAT I AM," (Exodus 3:14) or as the later ancient Greek translation known as the Septuagint would read, "I am the one who is" or "I am the being one." The God of the Bible makes very clear His identity as the one, self-existent, eternal, living God. The very first verse of the Bible announces that it was He who created both the heavens and the earth, (Genesis 1:1). If we are to know a being as ultimate and wholly other as God, it can only be through God willfully making Himself known to us, and He has done so in the Bible, through His Spirit, and in the person of Jesus Christ.
Numerous fulfilled prophecies in the Bible and its accurate knowledge of the world and how it works provide us evidence of its trustworthiness and divine origin. The historical evidence for Jesus bodily Resurrection from the dead and for His other miracles also offer a compelling case that the God of the Bible is, indeed, the one true God. The God of the Bible is not just one culture's version of every other god. The great I AM of Holy Scripture is the unique Creator, Governor, Provider, and Redeemer to whom all men of every nation must come, and on His terms, not on our own. While this might be "narrow" and "exclusive," it is true and is, in fact, quite good news! God has made Himself known to His creation and provided a way for us to come into true fellowship with Him! Not only is there one true and living God to believe in, but you can also know and commune with that God! (See our article What is the Gospel).