Monotheism is the belief in one true God who is the only creator, sustainer, and judge of all creation. Those who believe in one God do not need to have it proven to them. But then again, those who believe in polytheism (many gods) or atheism (no gods) probably don't need their beliefs proven either. Nevertheless, arguments for monotheism abound, and include revelatory (from Scripture), philosophical and historical anthropology.
Is there proof for monotheism?
Biblical arguments for Monotheism – The Bible is absolutely clear that there is only one God: Deuteronomy 4:35: "To you it was shown, that you might know that the Lord is God; there is no other besides him." Deuteronomy 6:4: "Hear, O Israel: The LORD our God, the LORD is one." Malachi 2:10a: "Have we not all one Father? Has not one God created us?" First Corinthians 8:6: "yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist." Ephesians 4:6: "one God and Father of all, who is over all and through all and in all." First Timothy 2:5: "For there is one God, and there is one mediator between God and men, the man Christ Jesus." James 2:19: "You believe that God is one; you do well. Even the demons believe—and shudder!"
However, to simply say that there is only one God because the Bible says so is, for many, an example of circular reasoning. If the Bible is not the Word of God (2 Timothy 3:16), then anything it says about God is not to be trusted. However, since the Bible has been preserved supernaturally down through the millennia, and it also has plentiful supernatural evidence confirming what it teaches, the argument for monotheism is greatly strengthened. There is alo the Person and work of Jesus Christ, whose miraculous birth, life, and the miracle of His resurrection proved that He was God (or at the very least approved by God) because God cannot lie or be deceived; therefore, what Jesus believed and taught was true, including monotheism.
Historical arguments for Monotheism – There is no doubt that monotheism has had a vast effect on the religions of the world. From the beginning of time, the most influential religions of the world have the concept of one God in common—a monotheistic God who is personal, masculine, lives in the sky, has great knowledge and power, created the world, is the author of a morality to which we are accountable, and whom we have disobeyed and are thus estranged from, but who has also provided a way of reconciliation. Virtually every religion carries a variation of this God at some point in its past, even those which eventually devolved into the chaos of polytheism, animism, or magic. Religions simply do not begin with chaos and develop into monotheism, with the very rare exception of Islam, which has come full circle back into a monotheistic belief. Even polytheism is often functionally monotheistic or henotheistic (one God over many). It is a rare polytheistic religion which does not hold one of its gods as sovereign over the rest.
Philosophical/Theological arguments for Monotheism - There are many philosophical arguments for the impossibility of there being more than one God in existence. First, if there were more than one God, the universe would be in disorder because of multiple creators and authorities, but it is not in disorder; it is perfect order and therefore argues for only one God. Second, since God is a completely perfect being, there cannot be a second God, for they would have to differ in some way, and to differ from complete perfection is to be less than perfect and not be God. Third, since God is infinite in His existence, He cannot have parts (for parts cannot be added to reach infinity). If God's existence is not just a part of Him (which it is for all things which can have existence or not), then He must have infinite existence. Therefore, there cannot be two infinite beings, for one would have to differ from the other.
Can the existence of God be proven?
What is the Trinity?
Deism - What is it? What do deists believe?
What is atheism?
What is agnosticism?
Truth about God