Let’s start with Hitler’s objectives (it is a book by Adolf Hitler, after all). He was born, baptized and raised Roman Catholic; however, by the time the Reichskonkordat was signed his religious beliefs were very different from Catholicism.
To put it very simply, he believed in (1) the existence of Supreme Power that he referred to as “Providence”; (2) the immutable and ultimately omnipotent Laws of Nature established by the Supreme Power; (3) in his Divine Mission in service of that Supreme Power; and (4) in being protected by this Supreme Power as long as he is fulfilling his Mission.
He believed in his very personal and very direct relationship with this Supreme Power and thus had no need for an intermediary between him and Providence. Consequently, he did not need to belong to any Church – not Catholic, not Protestant, none.
True, in his childhood he was mesmerized by the magnitude and splendor of the Catholic Church (and even at one time was thinking about entering a monastery with the ultimate objective of becoming an abbot); however, during his later years (especially the ones spent in Vienna) he became disillusioned in Christianity – to the point of considering it a “false religion” and even the “Jewish invention” (an important component of a “Jewish conspiracy” against the “Aryan race”).
However, his view of a Catholic Church (or any church for that matter) was entirely pragmatic. A textbook national-sociopath, he considered every church and every religion – Catholic, Protestant, Orthodox, etc. – a mere tool to be used to achieve his genuinely grandiose objectives.
Adolf Hitler loved his country and his people deeply, sincerely and passionately; hence he was firmly committed to (a) healing the economic ills of Germany – first, and foremost, unemployment; (b) radically increasing the living standards – and the overall quality of life of all Germans; (c) return all lands taken away at gunpoint by the “Versailles criminals”; (d) unite all German Volk into Ein Reich; (e) transform the defeated, robbed, depressed and economically devastated nation into a global economic, political and military superpower; and (f) make the Germans a genuinely happy nation.
He knew, of course, that to achieve these seemingly impossible objective, he needed every German to make the maximum possible contribution to his grandiose project.
And he needed both the Catholic and the Lutheran churches – the dominant religious forces in Germany – to inspire and otherwise motivate the Germans to maximize this contribution. In other words, he wanted every Christian church in the country to be a part of the solution to the immense problems faced by his beloved Germany – not another problem that had to be solved.
Now let’s see what were the objectives of the Holy See (more precisely, the Pope Pius XI on whose behalf the Reichskonkordat was signed and Eugenio Pacelli – the future Pope Pius XII who wrote and signed the document).
Unlike just about all political leaders of “democratic” nations (“democracy” is but an illusion now and was definitely an illusion then), both Pope Pius XI and his Secretary of State knew for a fact that (1) the Bolshevist Soviet Union was an existential threat to the Christian (i.e. Western) civilization; and (2) that the Nazi Germany was the only power capable of stopping the “Red Plague” in its tracks.
Unlike in a Cold War where the risks of an all-out “hot war” with the Soviet Union was precisely zero (thanks to a “mutually assured destruction”), in the 1930s and 1940s such a war with Bolshevism was inevitable.
And in such a war a totalitarian ideology (i.e. Bolshevism) could be stopped only by another totalitarian ideology (i.e. National-Socialism). The “democratic” ideologies were way too soft to successfully stand up to totalitarian ones (let alone defeat them).
Consequently, the fundamental objective of both Pope Pius XI and Eugenio Pacelli was identical to the ultimate objective of Adolf Hitler. Both sides wanted the Nazis to transform Germany into an economic and military superpower, able and willing (for whatever reason) to attack and defeat (ideally, destroy) the Bolshevist Soviet Union and thus save the Christian Churches (all of them) and the whole Christian (i.e. Western) civilization. Hence it is no surprise that they negotiated and signed the Reichskonkordat so quickly.
However, both leaders of the Holy Roman Catholic Church (#1 and #2 in the Church hierarchy respectively) were not only the lawyers and politicians (although both undoubtedly were).
They were also spiritual leaders, ordained priests, ministers, pastors who were supposed to take good spiritual care of their Catholic flock. And took this responsibility very, very seriously.
Although they were (understandably) up to their ears in very worldly politics, they still knew that their primary objective (for which they will judged by the Almighty God when their time comes) was to bring as many souls as possible to Heaven (either directly or via Purgatory). In other words, save as many souls as possible from Hell. Including German souls, of course.
According to Catholic doctrine (regardless of what the heretics in Vatican preach these days), the only way for an individual to ensure the salvation of his or her soul in the afterlife is to (1) belong to the Holy Roman Catholic Church; and (2) follow the Catholic teaching as close as humanly possible – on a “best effort” basis, so to speak.
Consequently, both Church leaders wanted from the Reichskonkordat (or from any concordat for that matter) the right to preach and teach the Catholic faith (converting non-Catholics to Catholicism, keeping Catholics in the Church and motivating them to be “good enough Catholics” at all times) and to structure and manage the Church in the country in question in the most efficient way – from the abovementioned perspective.
Obviously, they also wanted every catholic in Germany (actually, in every country) to have the right and the ability to follow the Catholic teaching – even when it contradicts the laws and the decrees of the government in question.
Unfortunately, wanting something and being able to make it happen are two different matters entirely. To make your desires a reality (i.e. to make the other side agree to them), you must have some kind of leverage in the negotiations in question.
And that’s precisely what the Holy See did not have – but Adolf Hitler did. The Vatican needed the Third Reich to avoid being invaded, conquered and destroyed by the Bolsheviks (a very real possibility even after the war – let alone before).
And Hitler did not need the Holy See – or even the Holy Roman Catholic Church for that matter. In fact, as his ideology (national-socialism) was not exactly compatible with Christianity (to put it mildly), Hitler would not have minded at all if someone just annihilates the whole thing, clearing the ground for the total and final victory of Nazism.
Hence it is no surprise at all that Adolf Hitler got pretty much all he wanted from Reichskonkordat. And although on paper it might have looked that the Holy See achieved its objectives (although it not always did), in reality it was on paper only.
For a very simple reason – the Führer had no desire to honor the terms of Reichskonkordat (with the exception of those cases when it was in his interests). Hence, if Pius XI and his Secretary of State hoped that Reichskonkordat would prevent (stop, actually) the persecution of the Catholic Church in Germany (and they probably did), they were in for a very rude awakening.