One basic goal would be to make the book palatable to atheists (in spite of the obviously baiting title). This goal would be achieved by striving for accuracy, comprehension and exhaustiveness in the representation of atheist arguments, avoiding weasel words and incorporating criticisms by atheist editors.
One point such a book would make is that meaningful discussion ultimately must revolve around atheism rather than theism, because there are many theisms, but there is only one atheism. Some will counter that there are, in fact, plural atheisms, e.g., Buddhism, but the "new atheists," sometimes called Western atheists, I expect would deny a true plurality of atheisms, and I want to address Western atheists on their own terms.
Anyway, it is easier to have a meaningful discussion about the truth/falsehood of atheism because it permits us to bracket the question of the truth/falsehood of any number of theisms. If atheism is false, and it competes with 1047 possible theisms, it may well be that 9.9̅ x1046 possible theisms are also false. But because atheism involves a finite set of existent conceptions of the universe, they can be more or less addressed in a single volume.
This is important, because the central theistic argument that I expect the book to rely on is radical contingency, and radical contingency itself says little to nothing about what sort of theism might actually be true. In fact, radical contingency could be true, and most anti-religious propositions could nevertheless remain intact. I have met anti-religious individuals who accepted the radical-contingency argument. That is, they accepted the necessity of an infinite, ultimate "being" or "substrata", but did not accept this entity's relevance to ordinary life.
Which, so far as the scope of this book would be concerned, is fine. In spite of the title, the goal here is not conversion to Christianity or even to any religion at all. The reason for a book like this is to be a repository of the logical interaction of arguments. Nevertheless, I do have some theological motivation. A common misconception among Catholics, and people in general, is that belief in God's existence is a matter of faith. Catholicism does not teach this. Where strictly God's existence is concerned,
God, the source and end of all things, can be known with certainty from the consideration of created things, by the natural power of human reason: ever since the creation of the world, his invisible nature has been clearly perceived in the things that have been made. (First Vatican Council, Dogmatic Constitution on the Catholic Faith, Chapter 2)So that's one thing.
Now, I could go beyond the strict scope of the book, and I may have to. The "new atheists" following Richard Dawkins stress more the inconsequentiality of all possible deities rather than their absolute absence, i.e. the teapot argument. So it might not be enough to show the plausibility of the "radical contingency" argument. Not sure.