Monotheism describes a belief system that accepts that there is only one God, a deity above all creation. All other gods are just lifeless statues with fancy names and immoral lifestyles. No prayers or incantations will make them talk to you and, if you think they have, you're delusional in all departments.
The adherent to monotheism must accept as a tenet that God did nothing for people for the entire Polytheism era, from 4004 BC through 1900 BC: He created the world and then buggered off for two millennia.
A false start
In Ancient Egypt, it was Pharaoh Akhenaten who insisted there was only one God. He promoted worship of the Sun Ra above the Moon Ra and even the Brisket Ra; above all the other Egyptian Gods. He even denied the other deities' temples their state funding.
Things got ugly when he started to insist that the Sun Ra was the only deity to worship. His advisors and prime ministers concluded that he had gone insane and conspired to overthrow him. The conspiracy was successful and, as this was before the invention of prisons, it resulted in his death, at which point the Egyptians returned to their customary polytheism.
Around 1900 BC, Abraham upped tent from Iraq, land-grabbed in a place called Canaan, and invented monotheism. At that point, it comprised only attempted human sacrifice and an extended holiday in Egypt. However, it took hold, its dramatic stories leading to the creation of the Bible, Jesus, Mohammed, and eventually The Beatles.
Moses was regarded as the go-to prophet. God gave Moses two lightning-carved stone tablets on top of Mount Sinai. These Ten Commandments lasted about ten seconds. Moses broke both of the holy relics after he returned to camp and found "his chosen people" cavorting and dancing around a golden statue dedicated to a plate of veal cutlets. In fairness, portions had been unusually meager since the move to the desert. Fortunately for monotheism, the text has mostly been re-created.
In addition to the movers of monotheism, there were Shakers. These monotheists were a splinter group from Quakers, organized in England and the United States in the 18th century. Their nickname owes to their odd behaviour during worship services, now known as Parkinson's Disease. The best-known minister of this monotheistic sect is Shakin' Stevens. The sect has been in decline since Stevens' time, as too many visitors to the congregations were shaken, not stirred.
Christianity claims to be the largest monotheism in the world. However, no sooner than it was founded, it lurched back toward polytheism, deciding that Jesus and the Holy Spirit were also divine. Peace was restored when clerics ruled that all three gods were the same entity, a sort of Holy Trinity, like pinch God and Holy Spirit goes, "Ouch!" About 95% of Christians accede to this ruling, the other 5% ticking "Don't Know." Opponents call this "tritheism" (God of Threes).
Islam says they are the fastest growing monotheistic religion. This is because the Koranic method of converting unbelievers, while not necessarily increasing the number of Muslims, dramatically decreases the number of non-Muslims. Muslims claim "Last Wordism" as regards God's direct involvement in human affairs, via the prophet Mohammed.
Smallest of the monotheistic religions, Jews are still convinced that God was only talking to them. (Christianity and Islam stole their copyrights.) The hallmark of Judaism is elaborate rituals, involving outrageous apparel such as the yarmulke and tefillin to be worn during prayers, without which God would not know you are praying.
This niche monotheism is a splinter group from Islam, dropping some of the more onerous requirements of Islam, such as pilgrimmages and suicide bombings. Rather than face Mecca during prayers, Baha'i orient themselves toward the French Riviera.
Mormons (officially, Church of Latter-Day Saints, and Rodeo) are a comparable splinter group from Christianity. Like Baha'i, the Mormon church modernizes doctrine; instead of stone tablets, there were metal printing plates, which have gotten just as lost in the passing of time. However, the requirements of Mormonism actually got more onerous, approaching those of divorce: one-tenth of your pay, and one year of your life, generally spent in sub-Saharan Africa trying to teach the natives that having sex with babies doesn't cure AIDS. There is also a wicked big choir.
Secularism, Wikipedia tells us, is the "indifference to, or rejection or exclusion of, religion and religious considerations." Thus, secular monotheism is a single-God religion, hold the religion. It is a phenomenon like sexless orgasm.
The earliest secular monotheism was Deism. Deists accept for the sake of argument that a single supreme being exists, but will not go out on a limb as to whether it is God, Allah, Yahweh, or perhaps the cherry Bakewell. Whoever it is, they believe he created the universe, wound it up like a top, then went off on a really big errand, leaving the universe behind like a forgotten toy.
Isaac Newton is considered to be a Deist, as he believed the Universe ran on clockwork and not divine power. Even so, he never claimed to be an outright atheist, being always mindful of what they did to Galileo. But he did think the Trinity was crap.
The Founders of the United States are often portrayed as vague Deists. This is because it is easier to get schoolchildren to look up to them when you don't disclose that they would have attended the church on the other side of the railroad tracks.
Humanism, or Secular Humanism, is another notorious non-religious monotheism. In Humanism, the humanist is the supreme being. This makes it especially easy for humanists to hold that God created us in His own image. Humanists pass time disparaging the trappings of all the other monotheistic creeds, while remaining members-in-good-standing of a church. They believe a sermon with a good scolding is equally effective as the threat of eternal fires of Hell. Albert Einstein was a notorious humanist, though analysing the implications in much less detail than stuff like the curvature of space.
The Unitarian-Universalist Church is a recent religion that holds the religion. Unitarians strive to provide church services, communions, collection plates, baptisms, and weddings while making the minimum number of demands on what the congregation actually believes, and none at all about what they do about it. A typical church service will invite a speaker from far away to give a moving presentation on Police hunting African Americans for sport. It will resolve to fight for exactly the right level of carbon dioxide in the atmosphere. The common denominator of this denomination is a congregation with far too much spare time on its hands.
A lucrative secular monotheism is Capitalism. The supreme being of Capitalists is money (which they call Capital). They say that, if you pray to Capital, you will become wealthy, just as long as you truly believe with all your heart.
The faith offering to Capital to prove one's belief must be in the form of Capital (or time recruiting followers — time, after all, is money). Adherents to Capitalism gladly make these monetary sacrifices, because they believe the money will be repaid several times over — a sacrament called Return on Capital — as the funds "trickle down" from the leaders. In the meantime, the leaders lead lavish lifestyles. A notorious Capitalist minister was Reverend Ike.
There are several denominations of Capitalism, which regularly war with each other over monetary policy: Church of the Dinero, Church of Moola, Church of the Yen, Church of the Rupee, Church of NYSE, Church of Dow Jones, Church of the Designer Brands, and more. Each competes strenuously for followers, because one can never have too much money.
Karl Marx created Communism and became the secular version of Moses; in place of stone tablets, Marx had doorstop-sized volumes Das Kapital, Das Kapital 2, and Das Kapital: Revenge of the Swiss. The supreme being is the proletariat (the new "chosen people"), who were locked into wage slavery. Communistic monotheism later claimed the Russian version was the true faith. Since the demise of that sect, Communism remains strong and in power in China, North Korea, and Congress.