They worshipped Idols and statues
Which religion did Prophet Mohammad's parents follow?
Not sure that it matters, really.
His father died about 6 months before he was born.
He was sent to live with nomadic relatives, shortly after he was born. And his mother died when he was about 6.
So he really didn't get any religious teaching from his parents.
It's actually hard to say, the reigon had been morphing from polytheism to monotheism, and was most likely influenced by the christian and jewish religions, so it's possible the people around him worshipped a facsimile of the jewish, christian or zoroastrian God. Here's an article about the relationship of the torah-bible-quran.
Religion is the number one killer in the world.