This is not the place to ask this question - I doubt there are many certified music historians here. You'd do better off at /r/metal or /r/truemetal or some place like that. Here's a good FAQ that /r/truemetal links to.

That said, it came from the music's origins, really. Black Sabbath (commonly cited as the first heavy metal band) were looking for something to set them apart. Being the odd bunch of people that they were, some of them were already into occult stuff like Aleister Crowley (Bill Ward especially, if I recall) and Ozzy was willing to do anything for a shock. The story that the band has told many times is that they saw people going into a horror movie and they said "Hey, those people are paying money to be scared! Why don't we do that with music?" The first Black Sabbath record has dark/gothic/satanic themes on it and the genre spread out from there. The early popularity of Alice Cooper's shocking, dark vaudevillian antics also helped propagate that theme.

My sources / good sources for further information:

Metal: A Headbanger's Journal

Heavy: The Story of Metal.

Those are both good documentaries on heavy metal history and both address this topic. Both are available on Amazon Instant Video. Ozzy's autobiography talks about this too. If you'd like, I can point you in the direction of some books on metal history.

One thing to realize about a lot of heavy metal, the "satanic" aspects are, more often than not, theatrics. Heavy metal as a whole could possibly be divided in two sets of themes, either theatrical or political. And while many bands definitely fall in to one or the other category, it doesn't mean that a band that predominantly does "theatrical" songs doesn't do "political" songs or vice versa.

Thrash metal, in general, tends to have more political songs, anti-war being a common theme. See: Metallica, Megadeth.

Death Metal, depending on sub genre, tends to be more theatrical, going the "horror movie" route, with songs about serial killers, blood and guts, extreme "violence", see bands like Cannibal Corpse, Mortician.

Power metal, tends to be overwhelmingly theatrical, but tends towards a fantasy theme. Knights fighting dragons, great battles, or even Lord of the Rings. See bands like Blind Guardian.

But if you want the bands that took heavy metal satanism to its logical extreme, you want black metal. Black metal began in Norway in the 80s, these were the people who burned churches down, who made anti-christianity their primary theme, reason, everything.

While it is arguable how many of them actually "believe" in the values they espouse, logic seems to dictate at least some of them do. Gorgoroth put on a show in the hometown of Pope John Paul II, that featured naked people with black bags over their heads tied to crosses, severed animal heads on stakes at the front of the stage, and much of the lighting by torches. (The concert is titled "Black Mass in Krakow" if you'd like to see pictures of it or read more about it, though you can also purchase the video now).

There are also other notable events like Varg murdering Euronymous, the singer of Mayhem 'Dead' committing suicide with a shotgun, then the band uses a picture of him dead on the floor as an album cover, as well as some people claiming they ate bits of his brain and used bits of his skull to make necklaces.

If you'd like to read more specifically about the rise of Black Metal, I'd recommend the book 'Lords of Chaos'. It primarily focuses on Black Metal, but it does go in to some of the history of heavy metal prior to that, and it is filled with interviews of many of the people involved in the early scene. Additionally, if you'd like to watch a documentary following a few people from the scene around, and them talking about and discussing the early days of Black Metal, look for 'Until the Light Takes Us'. Personally, I feel the documentary is best watched after reading the book, as some of the people talk about events in such a way as "you should already know the basics about this event".

The interest in the Satanic in heavy metal stems from rock music roots in the blues. The blues in full of imagery about the devil. One of the central myths of the blues is that Robert Johnson sold his soul to the devil in order to play guitar the way he did. You also have many of the most popular and influential early rock musicians such as Elvis, Little Richard and Jerry Lee Lewis being raised in fundamentalist Christian sects and felt conflicted by playing "the devil's music" as it was called by their churches. This is the bed rock on which Satanic heavy metal rests. English rock musicians like the Rolling Stones and Eric Clapton started their careers as blues musicians and became familiar with the work of the American blues musicians and early rock n' roll musicians and began covering the songs and spreading the imagery of the blues. The Rolling Stones who were marketed as the bad boy alternative to the Beatles where particularly important in popularizing the trend towards satanic lyrics with their 1968 hit "Sympathy For The Devil" or by naming their 1967 album "Their Satanic Majesties Request". Eric Clapton with Cream furthered the Robert Johnson myth with his epic cover of Johnson's "Crossroads" in 1968. Black Sabbath put out their first record in 1968 and Led Zeppelin in 1969. At the time the style of music these bands played started to be called hard rock by the music press to differentiate it from the more pop styles of rock. The runaway success of Led Zeppelin further spread the imagery of the blues and Satan to a worldwide audience. Jimmy Page's interests in the occult became quite well known among fans of this type of music. Page even bought Alister Crowley's house.

So that's is a rough sketch of the interest in Satan in rock music and how it became part of the heavy metal canon.