User blog comment:B214/Volume 21 info & spoilers/@comment-93.105.246.116-20160606130720/@comment-1359322-20160606140643

"“Long time ago, there was a three sided war between the Devils, the Fallen Angels, and God with its Angels followers. All three sides had a large army and they fought for almost eternity. As a result, all three sides lost most of their troops and the war finished several hundred years ago with no side winning.”"

- Volume 1

Also the Bible God didn't die immediately after sealing Trihexa, if he did then he wouldn't have been able to seal Ddraig and Albion in Sacred Gears. Ddraig and Albion interfered in between the Great War.

So God did join the Great War and only died several hundred years before DxD.

"“—It was the God from the Bible. That God sure was amazing. He found 666 even before any of us and put thousands of seals on him. Maybe that may have been the cause of death of the God from the Bible. After all, there were so many powerful seals that would be regarded as forbidden level that it wouldn’t be weird for a God to die after using those kinds of seals. So it’s a plausible story for the God from the Bible to vanish if he took part in the war between the Three Great Powers due to fatigue.”"

- Volume 16