What is the main conflict in Good Omens by Neil Gaiman?

Good Omens is the story of an angel and a demon who are both trying to do their jobs that are part of the Great Plan. Although Aziraphale and Crowley are adversaries by nature and profession, their relationship develops into a friendship merely because of the time spent together over thousands of years. In this unlikely pairing, compromises are made between the two of them so that they can both appear to be accomplishing their missions without overcoming the other too much. When the birth of the Antichrist occurs, they agree to work together and try to see if their influences on the child have any effect. By the time they locate the correct child, it is almost too late as Armageddon is about to begin. Events unfold and the world is saved. At the end, Aziraphale and Crowley wonder if their involvement had any effect.