With the benefit of hindsight, is there a moment in history that we can now look at and say with a reasonable degree of certainty that it was the moment Germany had truly lost. I am assuming that this would be a point well before the war was declared over. Was it some decision made, some key position lost?
It was December 1941 at the very latest. Without being able to knock out the Soviet Union in one campaign season (I don't think it was ever viable but still), Nazi Germany's defeat was virtually ensured due to the superior economic power of its opponents.
I wrote about this in some detail here:
Four events I would say are arguable here.
The battle of Moscow, a failure to capture the Soviet capital and decapitate it's government/central organisation. Though Germany was still very much still in the war, it never recovered it's momentum or threatened the USSR's central command again.
Attack on Pearl Harbor, and subsequent declaration of war on the US by Germany. Brings the USA into the war.
The battle of Stalingrad, the entire 6th army destroyed, the end of any attempt to capture the Caucuses and cut off Russia's main supply of oil. Huge propaganda and morale victory for the Soviets, and the inverse for the Germans.
The battle of Kursk. Decisive defeat, Germany would be on the defensive on the Eastern front for the remainder of the war. They would be retreating with the Soviets behind them all the way to Berlin.