The ascetic ideal appears in many forms: the philosopher's detachment from the body, the saint's self-mortification, the scholar's will to truth at all costs. In each case, something that affirms life is renounced in favour of something that seems to stand outside it. Nietzsche argues that these are all variations on a single strategy: the deployment of meaning against suffering.
The key figure in the ascetic ideal is the priest, who is paradoxically an instrument of life's self-preservation. When the herd is sick, suffering, and without meaning, the priest provides an interpretation of that suffering: you are guilty, you have sinned, your suffering is punishment. This answer is false, but it is an answer. It gives suffering a meaning, and meaning is enough to make suffering bearable.
The deepest paradox of the ascetic ideal is that it is itself a will, a form of power. The very will that drives asceticism, however self-denying, is not the absence of will but a will for nothingness. And a will for nothingness is still preferable to no will at all. This is the closing argument: humanity has submitted to the ascetic ideal for so long because it would rather will nothing than not will. The question left open is whether a different ideal can replace it.
The Third Essay identifies science, not religion, as the contemporary heir of the ascetic ideal: the scientist's will to truth at any cost, including the denial of all metaphysical comfort, is itself an ascetic posture.