The most casual reading of the Bible reveals that there can be only ONE supreme self-sustaining being. This is the core belief of Israel which distinguished them from all the polytheistic nations around them.  There never has been, nor ever will be more than one, eternal, self-existent, and autonomous being.  Although others many CALLED gods, there is only one REAL God by nature.  God has no equal.  God has no one he is accountable to. There is no bigger God over Him, and no one he needs to consult with to do anything.  No other being anywhere has sufficient power to prevent God from accomplishing WHATEVER he chooses to do--whenever and however He chooses to do it. 

"He is the irresistible force; there are no immovable objects."

 Deuteronomy 4.35 -- You were shown these things so that you might know that the Lord is God; besides him there is no other.

 Deuteronomy 4.39 -- Acknowledge and take to heart this day that the Lord is God in heaven above and on the earth below. There is no other.

 Deuteronomy 6.4 -- Hear, O Israel: The Lord our God, the Lord is one.

 Deuteronomy 32.39 -- See now that I myself am He! There is no god besides me. I put to death and I bring to life, I have wounded and I will heal, and no one can deliver out of my hand.

 2 Samuel 7.22 -- How great you are, O Sovereign Lord! There is no one like you, and there is no God but you, as we have heard with our own ears.

1 Kings 8.22-23 -- Then Solomon stood before the alter of the Lord in front of the whole assembly of Israel, spread out his hands toward heaven and said: "O Lord, God of Israel, there is no God like you in heaven above or on earth below--you who keep your covenant of love with your servants who continue wholeheartedly in your way."

1 Chronicles 17.20 -- There is no one like you, O Lord, and there is no God but you, as we have heard with our own ears.

Psalm 83.18 -- Let them know that you, whose name is the Lord--that you alone are the Most High over all the earth.

Psalm 86.10 -- For you are great and do marvelous deeds; you alone are God.

Isaiah 43.10  "You are my witnesses," declares the Lord, "and my servant whom I have chosen, so that you may know and believe me and understand that I am he. Before me no god was formed, nor will there be one after me. I, even I, am the Lord, and apart from me there is no savior. I have revealed and saved and proclaimed--I, and not some foreign god among you. You are my witnesses," declares the Lord, "that I am God. Yes, and from ancient days I am he. No one can deliver out of my hand. When I act, who can reverse it?"

Isaiah 44.6-8 -- "This is what the Lord says--Israel's King and Redeemer, the Lord Almighty: I am the first and I am the last; apart from me there is no God. Who then is like me? Let him proclaim it. Let him declare and lay out before me what has happened since I established my ancient people, and what is yet to come--yes, let him foretell what will come. Do not tremble, do not be afraid. Did I not proclaim this and foretell it long ago? You are my witnesses. Is there any God besides me? No, there is no other Rock; I know not one."

Isaiah 44.24 -- "This is what the Lord says--your Redeemer, who formed you in the womb: I am the Lord, who has made all things, who alone stretched out the heavens, who spread out the earth by myself…."

Isaiah 45.5 -- "I am the Lord, and there is no other; apart from me there is no God."

Isaiah 45.18 -- For this is what the Lord says--he who created the heavens, he is God; he who fashioned and made the earth, he founded it; he did not create it to be empty, but formed it to be inhabited--he says: "I am the Lord, and there is no other."

Isaiah 45.21 -- "Declare what is to be, present it--let them take counsel together. Who foretold this long ago, who declared it from the distant past? Was it not I, the Lord? And there is no God apart from me, a righteous God and a Savior; there is none but me."

Isaiah 45:22 -- "And there is no God apart from me, a righteous God and a Savior; there is none but me. Turn to me and be saved, all you ends of the earth; for I am God, and there is no other."[Isaiah 45.21-22]

Isaiah 46.5 -- "To whom will you compare me or count me equal? To whom will you liken me that we may be compared? Some pour out gold from their bags and weigh out silver on the scales; they hire a goldsmith to make it into a god, and they bow down and worship it. They lift it to their shoulders and carry it; they set it up in its place, and there it stands. From that spot it cannot move. Though one cries out to it, it does not answer; it cannot save him from his troubles. Remember this, fix it in mind, take it to heart, you rebels. Remember the former things, those of long ago; I am God, and there is no other; I am God, and there is none like me. I make known the end from the beginning, from ancient times, what is still to come. I say: My purpose will stand, and I will do all that I please. From the east I summon a bird of prey; from a far-off land, a man to fulfill my purpose. What I have said, that will I bring about; what I have planned, that will I do."

Mark 12.28-29 -- One of the teachers of the law came and heard them debating. Noticing that Jesus had given them a good answer, he asked him, "Of all the commandments, which is the most important?" "The most important one," answered Jesus, "is this: 'Hear, O Israel, the Lord our God, the Lord is one.'"

1 Corinthians 8.4 -- …about eating food sacrificed to idols: We know that an idol is nothing at all in the world and that there is no God but one. For even if there are so-called gods, whether in heaven or on earth (as indeed there are many gods and may lords), yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live.

Ephesians 4.4-6 -- There is one body and one Spirit--just as you were called to one hope when you were called--one Lord, one faith, one baptism; one God and Father of all, who is over all and through all and in all.

Galatians 3.20 -- A mediator, however, does not represent just one party; but God is one.

1 Timothy 2.5 -- For there is one God and one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all men--the testimony given in its proper time.

1 Timothy 1.17 -- Now to the King eternal, immortal, invisible, the only God, be honor and glory for ever and ever. Amen.

James 2.19 -- You believe that there is one God. Good! Even the demons believe that--and shudder.

...there can be ONLY ONE! -- The Highlander

The problem arises when people begin describing who they think that God is!  Rather than submitting to the revealed truth of Scripture, people frequently define some "God" who suites their own needs and desires.  Their "God” ends up simply being an extension of their own opinions and imaginations--rather than the real God who bjectively reveals Himself in the Bible. 

That is why I can claim to love and serve "God" while someone from the Watchtower, Mormons, Catholics, Christian Scientists, etc.  can claim the same thing with equal sincerity.  But sincerity  has never been the issue!  The issue is whether that "God" we believe in is the real God who can save us?”  If  not, we're lost in our sins.

One writer illustrated the logic this way: 

Ice on a frozen lake has an objective, measurable, and verifiable thickness.  No matter how sincerely anyone BELIEVES that the ice will support their weight--it just can't if it's only ¼” thick.  Conversely, if that ice is actually 4’ thick it can hold up several cars--whether you believe it or not!  Belief--no matter how popular or sincere--doesn't affect the outcome.