I read somewhere, perhaps on Arab News, one of my regular daily reading and news sources, a very interesting explanation by a Muslim scholar. He basically compared the three religions of Abraham, in the order of their occurrence, with attempts by God during a "day" of salvation. So he starts out with the Jews and at first they obey, and then by late morning they drift away, so God next sends Jesus, and at first they are all present, but then by afternoon they drift away, and so he sends the Prophet (PBUH). I wish I bookmarked the link because it's much better if you read it yourself than the way I've summarized it, but I think you get the drift. This is a very good analogy and worth contemplation for a number of reasons. One is that the author recognizes that God is consistently sending the same message through all three faiths. In other words, it is the same God who is trying to provide awareness of him and hence the means for obedience and salvation. Thus while the specifics are different (and that fits with the analogy of the day lengthening and drawing to a close), nonetheless it is the same and one, only true God who sends prophets and draws people closer in order to provide the "true-ness" of faith and accurate knowledge of God. And like all good analogies, it takes the not understandable and puts it in a frame of reference that people can and do understand.
However, with all analogies related to God, by putting God into even a worthy and good analogy there is inadvertent trimming away of much of God's abilities and "motivations." So this analogy makes a great foundational discussion point, and I am sure that it will help make mental light bulbs go off. Equally important though is to recognize where the analogy does not and cannot completely communicate all of God's activities and will.
For example, the analogy has no room to mention that God does have an existing on going relationship with every human who has ever lived, and who lives today, even those outside of the three faiths. God constantly seeks to be known through the Holy Spirit, even to those who deny his very existence. We have proof of this because we know that every human has a guardian angel. One cannot have a guardian angel without having a "built in" path of access to God and grace, and whether a person uses it or not, the channel is always open from the side of God. Jesus mentioned this when he warned that people should be kind to children because the children's angels constantly "face God." So whether one is within the three faiths or not, every individual person has access to God in potentiality.
So by taking this knowledge we need to recognize that God is simultaneously inviting all people to know him, love him and serve him, to have as pleasant, pious and charitable life as possible, and therefore reach salvation at the end of one's life. If you accept this truth then you must recognize two things. One is that God has a different measure of "success" than humans. God has a soul by soul person by person level of success. Therefore he would not view a "poor turnout" to the banquet of Judaism or Christianity as a "failure" requiring "going back to the drawing board." People who remained true to their faith under Communism would be an example of where God would not write off the entire country or that faith as a "failure" so long as there is even one person who remained true. So the second thing that one must recognize about God is that God does not work "sequentially." Just because the three great opportunities to better understand the Kingdom of God arose over a human defined sequential time period (Israelites first, Christians second, Muslims third) does not mean that God "thinks" or "operates" sequentially. God is constantly reaching to each and every person through all time via the Holy Spirit. God "has" to reach each person within the reality of where they are living and what the societal structure is where they live. Again you could look at the extreme past of Communism as an example of where Jews, Christians, and Muslims were all oppressed and all had difficulty accessing resources of their own faith, but that does not mean that any of the three faiths were "a failure."
I think this was a great example, which is why I have been thinking about it for two days, and repeat it here, because like I said, it is a very useful foundational analogy to make the greatness of God understandable in a human context. That is "part one" though, because "part two" means that as we did here, you must consider how the limitations of human comprehension do not limit the truth of what God does and how he does it.
I hope you find this discussion useful.