According to the Bible Jesus will return at the end of times to save all the good souls. As believed it's going down for a lot of people by then because no one surely knows wether they are going to heaven or not.
By saying "Jesus Back", you simple mean to say that "it is going down", "It's on" or Shit's is on". Pretty much things in that nature, the meaning of Jesus Back varies per sentence, you'll have to be sharp to catch what the person meant when he/she says the word. "Jesus Back" can be used in both a negative way and a positive way.
This word is also hashtaggable as #JesusBack
Girl: "I just wanted to tell you that I got Chlamydia and Herpes"
Guy: "Yo, Yo, Yo what you say? You better hope I ain't got that shit because if it's so it's Jesus Back on yo ass"
(Probably how the Rihanna and Chris Brown convo went down before he got the test results back)
Guy #1: "Yo I just saw Ray Ray drunk as fuck yelling at a bunch of 5-0 and telling them they need to go cornhole they momma's....We need to go help him brotha!"
Guy #2: "Naw cuz Fuck that! Jesus Back! He's definitely gonna get Rodney King'ed now and I ain't getting my ass beat for no snitch. Let's move out before the coppers mistake us as his friends and busts some caps in ours asses like we're Sean Bell"
Girl #1: "Girl I just won 20 million dollars by playing lottery"
Girl #2: "Oh girl Jesus Back, Oh lord, Jeeeesuuuus Back!"