Rather than a general theory or explanation of either economics or religion, Weber attempts to draw a specific link between what he sees as the conjunction of the work ethic of Protestant (mainly Calvinist) spiritual teachings, and the success of Western European Capitalism.
Weber is an astute analyst, in many ways. He rightly notes that often the 'sine qua non' of Capitalism is thought of as "greed". Arguing against this notion, Weber points out that all societies have had greedy people within their particular economic system-greed is thus a factor irrespective of economic systems. Replacing this, Weber proposes that the "spirit" of Capitalism be thought of as a particular moral attitude towards work and idleness-an attitude that holds that constant and diligent work for its own sake is a moral imperative. In the face of what Weber calls "the radical elimination of magic from the world" this work ethic was the existential option left for people in terms of atonement and personal compensation for inadequacies. I believe that these two insights are right on target.
If there is a weakness involved in his characterization of this Protestant "Ethic," it lies in the fact that Weber attempts to draw a strict dichotomy in the origins of this ethic. He states forcefully that this ethic does not come out of any Enlightenment thought. The problem with trying to separate this ethic from the Enlightenment, is that this ethic which posits diligent work for its own sake is clearly found in the ethics of Immanuel Kant, who classified this kind of work and labor as a "duty" (ethical rule) that the self has to itself. In other words, how much of this is the legacy of the Reformation and how much of this is the legacy of the Enlightenment?
The necessity for this kind of work also appears in the ethics of Hegelian philosophy. Hegel characterizes work as a means of the realization of Spirit within the human self, since the performance of duties which one would not normally choose to do can be thought of as a deliberate placing of oneself in the context of alienation. The individual then, through diligent "work," attempts to convert that which is foreign (antithetical) to the self into that which is of the self. Work is thus a means of overcoming a system of deliberate self-alienation, and is vitally necessary. Kant and Hegel, clearly two giants of Enlightenment thought, both maintain that the essence of diligent work is to become, not acquire-acquisition is a by-product and consequence of work. This is very similar to Weber's characterization of this ethos.
Another problem arises when we attempt to draw a strict separation between the worldly attitudes of Catholic monasticism and this "Protestant Ethic." While it is certainly true that Catholic monasticism placed a high degree of value on contemplatio, Catholic dogma, from Augustine through Gregory the Great and onwards, held explicitly that one must always return to work in the world-contemplatio was always insufficient in itself as a mode of being. Biblically, this was often seen in light of the Hebrew story of Rachel and Leah, as well as the Greek story of Mary and Martha. The contemplative life is certainly of "higher" value in Catholic thought, yet it must be seen as returning the soul to the life of activity, lest the soul run the risk of the heresy of "Quietism." Some forms of Catholic mysticism ran into heretical issues precisely because they held that the life of activity should be abandoned. So, while there may be a difference in degree, we should be careful not to draw a stronger split than is there. Weber writes as if only Luther or Calvin has the concept of a life's "calling," when this was always already part of Catholicism too.
This entire issue actually has its roots in Greek political philosophy, where we see a clear tension between the "practical life", and the "contemplative life." The issue persists into Roman life. We can even see some evidence of this type of Protestant ethos in Stoicism, which held that the active pursuit of virtue and public activity was the highest good. Contrast to Epicureanism, which held that the private, quiet study of philosophy and other pleasures, away from worldly life, was the highest good. The issue, of course, reemerges in Christian thought. But for all of its force in Protestantism, we must not take a myopic view that this was somehow unique to Protestantism in Western intellectual thought. Other factors than religion must have also played a role in the development of capitalism.
The role of Judiasm is Weber's biggest problem. According to his own endnotes, Jews enjoy more economic success and motivation---so why would Protestantism give birth to Capitalism?
We should nonetheless congratulate Weber for attempting to take a close look at the interactions between religious and economic thought. Like Marx, his work serves a good framework to examine the way religious thought influences and inteacts with factors like world economics.