What is Monotheism?

The Abrahamic faiths originally all taught monotheism as their central tenet and two of them still do. Monotheism is the belief that there is only one God. Not a family of 'gods', no mother, no father, no son, no daughter etc etc. It also means no copulating with humans to produce demi-gods as some ancient religions believed.

In order to explain further I will break down the Islamic view of the three original Abrahamic faiths and compare with how they've evolved.

Judaism.

Judaism gets it's name from the tribe of Judah. Islam believes in the same lineage of prophets as jewish people do. ALL these prophets have professed monotheism as their central value so God originally chose these people to spread the word of monotheism and not commit idolatry. As a warning and an indication of the ease with which one can slip into statue worship is in the story of Musa {pbuh}, whereupon in the short space of time it took him to go up the mountain to receive the ten commandments, upon his return his people had already smelted down gold to create a calf statue to worship instead. The jewish people were liberated from bondage so that they could be free and spread the word of monotheism to other people of the time. So Judaism was a monotheistic message to the world, meant to be spread far and wide to ALL peoples and nationalities.
This then subsequently changed when people believed they were special because they were chosen. However being chosen for a task does not necessarily make you "more beloved" or "more deserving by default". For example if I choose one child from a class of thirty to go and post a letter for me, it isn't automatically because they are the "best" child or the most deserving. They may not be the fastest (in order to get there and back quickly) or even the most intelligent (in order to work out the best route). They have simply been chosen to do something. The choosing itself doesn't make them more worthy than everyone else. The only thing that would elevate them above everyone else is completing the task of posting the letter that was given to them (i.e. in the main example of professing monotheism to the world and persuading others to do the same).
However, this is what became of their faith. Rather than completing their task and welcoming others to monotheism, it became a racial religion, which only accepted themselves as "saved" and everyone else as "gentiles/goyim". Rather than proselytise monotheism to the world, it became something kept to themselves with the false belief that the act of choosing made them the special ones rather than completion of their task. They still retain monotheism but are not spreading that message to ALL peoples as they were instructed to do according to Islam and mistakenly believe they will automatically inherit heaven regardless of their actions.
They had therefore deviated from the message given to them hence the need for another messenger to "correct" their trajectory...prophet Isa {pbuh}.

Christianity.

Prophet Isa {pbuh}, aka Jesus, was also a strict monotheist. A challenge laid down by many Islamic scholars to our christian brethren is for them to point out precisely where in the bible that Isa {pbuh} himself asked anyone to bow down and worship him or where he professed he was the son of God. In the past 2000 years no one has ever been able to do so. Many will point out some verse from Galatians/Corinthians/Phillipians/Thessolonians etc. etc. but all of these were authored by Paul and became a part of the bible generations after Isa's {pbuh} time on Earth. We don't want to hear the word of Paul, we want to hear the word of Isa {pbuh} and nowhere in the bible did Isa {pbuh} himself ever say that he was the son of God. In fact, he openly preached the opposite - to uphold the monotheistic Judaic laws and spread then to all mankind (not one specific racial grouping).
Three hundred years later, when the council of Nicea was convened the Roman Emperor Constantine wished to incorporate Christianity as the state religion. The problem was that he was a pagan (he was only baptised into Christianity on his deathbed) and most of his subjects were polytheistic Romans, believing in pantheon of gods many of whom had daughters and sons, many of whom ate the popular food of pork/ham at the time, dealt in usury etc etc - in fact the very opposite of what Isa {pbuh} used to preach. How to get around the problem of allowing your population to continue practising it's own beliefs (e.g. polytheism, eating pork, being uncircumcised etc.) whilst asking them to follow someone who did none of these things (Isa {pbuh} was a monotheist, he never ate pork, he was circumcised etc)? Easy, add an addendum at a later date of teachings by a so called disciple (some of which are admitted to be nothing more than dreams they had) which incorporate your polytheistic beliefs and culture whilst saying they now supersede the actual documented beliefs of the central figure themselves!

So unfortunately modern day Christianity is not original Isa's Christianity at all but is fact the evolution of Greco-Roman polytheism, put in place by someone (Paul) who never even met him, a couple of generations after his death in order to allow the empire to accept it. Instead of being a prophet, Isa {pbuh} became the son of God and everything the Roman's did at the time became incorporated into it turning it into a new form of what they were already practising. They had become "Paulists". Don't just take my word for it, here are quotes from well known western theologists regarding this issue:

• Soren Kierkegaard, Danish Christian philosopher and theologian, observes in “The Journals”: “What Martin Luther, in his reformation, failed to realise is that even before Catholicism, Christianity had become degenerate at the hands of Paul. Paul made Christianity the religion of Paul, not of Christ. Paul threw the Christianity of Christ away, completely turning it upside down; making it just the opposite of the original proclamation of Christ.”
• Gene Savoy, American theologian and clergyman, declares in his “The Essaei Document”: “Paul’s Christianity is another matter. He taught a different kind of theology than that shared by the original disciples who were schooled under Jesus. Paul was the father of Pagan Christianity; a movement based on a concept completely foreign to Jesus. The teachings of Jesus the Messiah were overshadowed by the teachings of Paul.”
• Bishop John S. Spong, Episcopal Bishop of Newark, New Jersey, USA, writes in his book, “Rescuing the Bible from Fundamentalism”: “Paul’s words are not the Words of God. They are the words of Paul - a vast difference.”
• Thomas Jefferson, third president of the United States and author of the Declaration of Independence; writes in his “Letter to William Short”: “Of this band of dupes and imposters, Paul was the great Coryphaeus, and the first corrupter of the doctrines of Jesus.”
• Thomas Cosette, a Christian scholar, writes in “Hebrew Prophecies of the Coming of Paul”: “This man Paul hijacked what is called the church. But he can only keep those who do not love the truth. Those who still have conscience and will compare his teaching and his testimony to Y’shva’s and the prophets without granting Paul’s testimony (is) the Word of God but (is) just another man’s testimony in light of Jesus’ teachings. Then they will discover that Paul usurps the truth.”

Such a perversion of the original message of monotheism resulted in the need for another messenger to again "correct" their trajectory...prophet Muhammed {pbuh}.

Islam.

It is important to say that Islam does not preach a "new" religion. We believe this is the original unaltered religion that has ALWAYS been preached throughout time but subsequently kept getting changed by mankind. This has resulted in new prophets (there have been thousands of prophets sent to mankind over time) being needed to "course correct" their peoples back to original monotheism. Our belief is that Muhammad {pbuh} wasn't only preaching to the local Arabs though, his message was to the world as he was the last messenger. This universal message speaks to all humans and is why Islam is the fastest growing religion in the world. You can obviously learn about Islam in these webpages but suffice it to say Islam remains a monotheistic religion.
According to Islam, monotheism is not just believing in one God, but it is also not ascribing any power to anyone other than God (this is actually the single most important pillar of Islam known as Tawhid). God created the universe but does not reside within it. Thus if you believe a holy man has become enlightened and worthy of worship, that would be classed as polytheistic in Islam because a created being is being ascribed attributes only the non-created God (who is outside creation) has. Similarly, prayer to any idol means associating that which only God is worthy of (worship) to a man made object so would come under the category polytheism. In short, anything within the universe (i.e. created) cannot be treated the same as that which is outside the universe (God).
This also means he is not everywhere as some people believe. This concept of God being everywhere was introduced to the west by Orientalists in the early 19th Century borrowing from eastern traditions (New Thought). As per the rule above, God is not everywhere according to Islam, He is outside creation so Islam eschews these beliefs and remains monotheistic in nature.

 

Summary.

I can summarise by giving you a table of actual practises of various beliefs. You can easily see yourself which ones align with monotheism and which ones align with prophet Ibrahim's {pbuh} original teachings and those which don't:
   
This shows how Judaism changed from the original teachings resulting in the need for a prophet (Isa {pbuh}) to correct this. However they still firmly believe in one God and associate no partners with God meaning they are still monotheistic.
Unfortunately it also clearly shows that the subsequent "modern christians" are merely "modern Romans" (following Paul's teachings/dreams rather than Isa's {pbuh} actions/words) and no longer part of the monotheistic tradition. Islamically they are classed as polytheists within the same grouping as Hindus, Qadiyanis, Bhuddists, Sikhs, Yezidis, NOIs, Ahmadis, Baháʼís, Jains, Wiccans, Shintoists, Zoroastrianists, Taoists etc. etc. and should more accurately be described as "Paulists"".
The only 2 major religions left following monotheism are Judaism and Islam.