All human beings live in the same reality
We all are subject to the same natural laws (gravity, fine structure constant, etc)
There are certain human universals (we all need to eat, sleep, drink, things like funeral rites, personal names, trade, etc)
We all exist not as causeless causes but in relationship to -- g_d being 'human beings acting in harmony with each other and reality' (it is in this sense that both Theists & Atheists can be said to believe in the same Deity -- tho, of course, there will continue to be Theists who want the 'special sauce' and want to divide...but then, we ALL ARE SPECIAL SNOWFLAKES...you just gotta pay attention)
That written, there will be believers who will still try to divide, to break up the common human experience, to hold humanity back...
That books title should more accurately be "Allah: One Christian's Response" (seeing as its impossible for one person to speak for an entire fan club...)?
Truly, "Christians: Same g_d?" The answer is: it depends on who one asks.
There is more evidence for Mohammad existing than Jesus -- the NT writings around Jesus are far different than the writings of Mohammad -- they produce different effects (as seen in our culture). Thank goodness people are very adept at cherry-picking what they want to follow and what they don't want to (cf. Mohammad), though that is problematic (cf. Moist Robot hypothesis, herd mentality, the various cognitive biases, our love of status & our hierarchical nature...)
The believer has to ask themselves of what importance is it that anyone else's g_d or deity be the same as theirs? What does it accomplish, aside from being a potentially fun exercise (exercising the brain and the imagination)?
Never let others stoop you from investigating what you want :3