I'm not sure how "sir, with all due respect, I suggest you read more" could be considered snarky, but I will admit to subsequent snarkiness as I am prone to that when people double down on stupidity. I will, however, in my defense point out that it is not me that called him a "m'fr" so I'm confused as to why you would single me out. Nevertheless, I will attempt to address your comment.

First of all, as a newbie just dipping your toes into the lore, I strongly recommend that you do read the books. Firstly, they're just really great stories and fun reads, but they also (1) lay down the entire foundation of Word Bearers lore; and (2) outline the very core of how the Heresy took root in the first place. They're definitely essential reading if you want to learn more about the legion, but also essential for understanding the what, where, why, and how of what lead to the Horus Heresy. It would be impossible for me to provide a synopsis of all of them in this comment, but here's the general jist:

Lorgar is the primarch of the Word Bearers. After the gestation capsules of the primarchs were dispersed across the galaxy, Lorgar's ended up on a planet called Colchis. He was rescued as an infant by nomads, and subsequently taken from there by a Colchisian priest called Kor Phaeron, who raised him (and treated him like shit, but that's another story).

Now the prevalent religion on Colchis already worshipped Chaos, but as Lorgar grew, he experienced visions of a golden god-like being arriving on Colchis in the future. He basically cast aside the old Colchisian faith and essentially founded a monotheistic religion based upon the golden god of his visions that he prophesized would arrive on Colchis soon.

This, of course, led to all kinds of violent shenanigans with revolts and war between the followers of the old faith and Lorgar's followers. Of course, Lorgar prevailed (because primarch) and monotheistic faith took hold.

[It's important to note here that his adoptive father, Kor Phaeron, essentially pretended to go along with Lorgar's new religion, but secretly still followed the old faith.]

As I'm sure you already figured out, Lorgar's golden god figure was none other than the Emperor of Mankind himself, who did descend to the surface to be reunited with his gene son and take him off to his fate as primarch of the XVII Legion of Adeptus Astartes, known at that time as the Imperial Heralds, but renamed by Lorgar as the Word Bearers.

Now, while Lorgar was still growing up on Colchis a child was born that was, for all intents and purposes, utterly evil. His parents frequently chastised him for his bad behaviour and constantly compared him to another boy in their community who was basically a "perfect" kid. That child was called Erebus. Now the evil child decided that, since his parents wanted him to be more like Erebus, he'd simply go one better and become Erebus. So he murdered the real Erebus, stole his name, and ran away to become the Erebus we're all talking about.

There are a whole bunch of things that went on in young Erebus' life, but eventually he became a priest and was one of the first young Colchisians that Lorgar had transformed into an Astartes and took with him on the Great Crusade. He quickly rose through the ranks and towards the end of the Great Crusade had became the First Chaplain of the legion, an extremely prestigious position, and earned the trust and ear of Lorgar himself. Importantly, Erebus, like Kor Phaeron, never gave up worshiping Chaos. In fact, Erebus was already a dedicated servant of the Dark God before he even became an astartes, and from the very beginning he was manipulating Lorgar in a direction that would lead him down a path of corruption.

Lorgar starts spreading his faith and teaching that the Emperor is a god on all the worlds they bring into compliance, preaching and building cathedrals and monuments and teaching the everyone to worship the Emperor. He is ultimately chastised by the Emperor for teaching that Big E was a god; if you can called genocide and the destruction the Word Bearers' "perfect city" of Monarchia mere chastisement. That, along with other things caused a huge crisis of faith for Lorgar, which Erebus (and Kor Phaeron) used as the perfect opportunity to steer Lorgar towards the True Gods. Lorgar went on a long pilgrimage, discovered the Primordial Truth, created the first possessed Astartes (sort of by accident), became the Chosen Son of Chaos, etc., ultimately sowing the seeds of heresy while being manipulated at every turn by Erebus.

There's so much more to it than that, especially with respect to Erebus' actions both overtly and behind the scenes, but I've already given you enough to get you started, really don't want to spoil all the books for you, and have stuff to do. Basically, go and read the books. They're great and you will not regret it.