A white gang member - "Ayy Jamal the nigga for real tho"
Jamal - "For real my g homie ain't know wass poppin dawg like on god on momma on crip on blood on gang!"
On God is a phrase which is a way of saying that someone is telling the truth. Similar to another phrase such as "I swear to God." Other common uses are "On my mom," "On my grandma," "On my grandpa," "On gang," etc...
Person 1: I whipped out of that parking lot so fast, spinning the wheel, I 'bout flipped the damn car over. Then the police started following me, and I had to run away.
This is the way how you show that you actually mean something, that it is true. If someone can't say for real, for real on God then it means that it is not actually legit.