Actually, it’s the one thing that religions you classify as true religions agree on:
You might just as well have said “The only truth about God is: God loves tithing. All major religions recognize this, it is when religions go further than this contradictions appear.” Mormons have tithing, the Scientologists have tithing, I seem to recall that the more “mainstream” Christians used to have it, too. Of course, nowadays they’ve gone beyond the basic tenet of God (that he really likes it when you give money to the church) and so have all split and schismed and appear to be different.
For the more bloody minded in the audience, we could also go with “The only truth about God is: God demands human sacrifice. All major religions recognize this, it is when religions go further than this contradictions appear.” Human sacrifice is what ties us together as a species. Throughout history major religions have incorporated human sacrifice. The Babylonians did it, the Etruscans and then the Romans did it. Druids did it. (It’s pretty much the only thing we know about them. They killed people. Oh, and also quite big on henges.) The God of Abraham just wanted to be sure people would do it, if he wanted them to. The God of the Christians did it, but just once and it was a different time then and he didn’t inhale. Polynesians? Check. The Incans did it. (And poor Xander almost paid the price for their sisn.) Kali and whatever the Mayan god’s name was had this thing where every time somebody was sacrificed in their name they got 10 frequent flier miles. Those two were later spotted in Otahiti sipping mai tais on the beach while getting a pedicure.
I don’t believe that all (or even most) major religions have had the whole “God/love” thing going on. It looks more like a sampling error due to the current prevalence of disinterested beard-in-the-sky gods that we seem to have recently. Was the fundamental tenet of the Babylonian pantheon love? The Egyptians, Greeks, Romans, Norse, Jews, Muslims, Hindus? Heck, was a fundamental part of those religions that somebody up there wants you to be warm and fuzzy?
This is the part where you respond that all the religions I listed aren’t real religions because they aren’t all about the love. That’s cool, but you’re arguing your way around a circle: “God is love => all true religion is about experiencing divine love => religions not about divine love aren’t real religions => the defining characteristic of religion is love => God is love”
But, because I like the cut of your jib, I’ll keep praying every morning for the sun to come up. I’d hate to ruin your picnic by plunging the Earth into eternal darkness. I don’t ask for any kind of fee or retainer for this, but tips are appreciated. I take cash but I also have PayPal.