As the story of Abram's encounter with God unfolds, one thing becomes incredibly clear: there are not actually multiple divine beings who equally deserve our worship. There are not millions of gods. There aren't even dozens of gods. There is only one, and His name is Yahweh.
Each of these faiths today points back to Abraham as their founder, but beyond that, they are very different from each other in several meaningful ways. The most meaningful difference is what they say about the most important character in all of history: Jesus Christ.
That last fact is what helps the theist looking for his God decide which God they ought to worship. Jesus' death was predicted hundreds of years before it happened, via a method of execution that didn't even exist at the time it was prophecied. Jesus foretold his own death and resurrection well in advance of its occurrence. When he bodily rose from the dead, which was widely and publicly attested, he demonstrated to all that He was God come as a man. He confirmed to all who would seek with an open mind that God had come to earth to show us who He was.