While he saw himself as a devout christian, he wanted to get rid of the old testament. It was his 'belief' that Yahweh was not a good person and should not have been related in any way to Jesus (he wrote before the bible was canonized). His opponents would easily defeat his arguments since it was Jesus that was born of the jews and made reference to fulfilling of jewish law.
Most people were illiterate those days and merely followed their leaders. People like Marcion dared to do something with his scriptures most people, who are not illiterate today, wouldn't do. He actually read them!
"I would just like to ask...Do any of the theists here believe in Satan/Lucifer/The Devil/Beelzebub etc...?
And why or why not?"
The same reason I disbelieve in gods,aliens visiting the earth and fairies at the bottom of my garden,to mention just three of thousands of things I disbelieve-----No evidence.
"Also could someone please give me a little information about such evil? "
Google is your friend and,Wikipedia is good place to check-perhaps begin by dropping the assumption that Satan is necessarily an evil figure.
Satan (Hebrew: הַשָׂטָן ha-Satan ("the accuser"); Persian "sheytân"; Arabic: الشيطان ash-Shayṭān ("the adversary") - both from the Semitic root: Ś-Ṭ-N) is an embodiment of antagonism that originates from the Abrahamic religions, being traditionally considered an angel in Judeo-Christian belief, and a Jinn in Islamic belief. Originally, the term was used as a title for various entities that challenged the religious faith of humans in the Hebrew Bible. Since then, the Abrahamic religions have used "Satan" as a name for the Devil.
Main article: Christian teaching about the Devil
See also: War in Heaven
In Christianity, terms that are synonymous with "Satan" include:
* The most common English synonym for "Satan" is "Devil", which descends from Middle English devel, from Old English dēofol, that in turn represents an early Germanic borrowing of Latin diabolus (also the source of "diabolical"). This in turn was borrowed from Greek diabolos "slanderer", from diaballein "to slander": dia- "across, through" + ballein "to hurl". In the New Testament, "Satan" occurs more than thirty times in passages alongside Diabolos (Greek for "the devil"), referring to the same person or thing as Satan.
* Lucifer is sometimes used in Christian theology to refer to Satan, as a result of identifying the fallen "son of the dawn" of Isaiah 14:12 with the "accuser" of other passages in the Old Testament.
* Beelzebub is originally the name of a Philistine god (more specifically a certain type of Baal, from Ba‘al Zebûb, lit. "Lord of Flies") but is also used in the New Testament as a synonym for Satan. A corrupted version, "Belzeboub", appears in The Divine Comedy.
* Satan is identified as the serpent who convinced Eve to eat the forbidden fruit; thus, Satan has often been depicted as a serpent. This interpretation goes back at least as far as the time of the writing of the book of Revelation, which specifically identifies Satan as being the serpent (Rev. 20:2).
* "The dragon" and "the old serpent" in the Book of Revelation 12:9, 20:2 have been identified with Satan. The Book of Revelation also refers to "the deceiver," from which is derived the common epithet "the great deceiver." Other terms identified with Satan include "the prince of this world" in the Book of John 12:31, 14:30; "the prince of the power of the air" also called Meririm, and "the spirit that now worketh in the children of disobedience" in the Book of Ephesians 2:2; and "the god of this world" in 2 Corinthians 4:4.
* Leviathan is described as "that crooked serpent", which is also used to describe Satan in Revelation 12:9. 'Sar ha Olam,' a possible name for Metatron, is described as Satan by Michael, Jehoel and St. Paul.
Satan is not just a Christian notion. The entire article is worth reading.
This amazing character is already prefigured in the Book of Job where Satan (in Hebrew “adversary”) seems to be God’s prosecuting attorney.
In Good Book, David Plotz writes:
“Here is what Satan is not: a fallen angel, wicked, omnipotent, demonic, living in hell, warring with God for dominion over the Earth, carrying a pitchfork, or dressed like an evil Santa. Here is what he is: argumentative, troublemaking. This is another example of the popular culture and Christianity oversimplifying and flattening a biblical character. Our modern Satan is a cartoonish incarnation of pure evil. The Bible’s Satan is fascinating because he’s ambiguous.”
From "Good Book," Harper Collins, New York, 2009, pages 248 & 249.