Having perceived that the spiritually healthy cannot look after the sick without becoming sick themselves, Nietzsche concludes that the sick need "doctors and nurses who are themselves sick." The role of the ascetic priest is precisely to tend the sick masses. He must be sick himself, but also strong enough to lead and dominate the masses. The ressentiment of the masses demands that they find someone to blame for their suffering, and this search for a scapegoat can be violent and dangerous. The ascetic priest serves the purpose of altering the direction of the ressentiment, by persuading the masses that they themselves, and no one else, are to blame for their suffering. This renders them harmless, promotes their self-discipline, and by organizing them into a religious framework of sin and guilt, helps to keep them apart from the healthy.
However, the priest only serves to ease the suffering of the sick without trying to cure the sickness itself. The sick are those who haven't the strength for the great struggle of humanity against its instincts, and religion does not give the sick strength so much as it eases their sense of displeasure at life.
Nietzsche identifies two primary ways in which the ascetic priest combats this prevailing sense of displeasure. First, there is the attempt to dull the sensations and the will so that the pain of this world isn't felt as keenly. The supreme state of redemption is seen, particularly by Indian philosophy, as liberation from everything: truth, knowledge, reality, good, evil, etc. all fade to insignificance as the soul slides into a deep sleep.
Second, there is the attempt to distract the mind from its suffering by means of hard work. The priest manages to convince the lower classes that hard work is blessed, and so they set themselves to it with ardor. Also, in commending small deeds of selflessness and neighborly love, the ascetic priest prescribes a harmless and easily attained expression of the will to power. This spirit of mutual helpfulness is what brings the weak together into congregations.
While these two are primarily "innocent" means of working against the feeling of displeasure, there is also the "guilty" means of working up an "orgy of feeling." Nietzsche admits that he could use a less negative term than "orgy of feeling," but asserts that he feels no inclination to soften his words for those who can't bear to hear them. These "orgies of feeling" are found in the concepts of sin, guilt, bad conscience and the like, and they are "guilty" means because they ultimately serve to make the sick sicker. The ascetic priest convinces the sick to find the cause of their suffering in themselves, to see their suffering as punishment. Once we come to see ourselves as sinners, there is no hope of being healed. Our suffering in that case is misunderstood as being fully our own fault, and we indulge this suffering in masochistic orgies of feeling.
Nietzsche remarks that the poisoning influence of ascetic ideals has also harmed good taste. For instance, Nietzsche finds the New Testament a despicable example of bad taste, a collection of anecdotes written by early Christians tacked on to the end of the Old Testament (which Nietzsche deeply admires) and then proclaimed to be its culmination. Nietzsche acknowledges that he is virtually alone in despising the New Testament, but he holds to his opinion.
In trying to understand Nietzsche's analysis of the "herd" (the masses who suffer and feel ressentiment), there are two fundamental facts to keep in mind. First, they are powerless. Second, like all things, their fundamental drive is the will to power. Because they are powerless, this will to power is re-routed and delayed at all turns, and yet they would rather (to borrow Nietzsche's phrase) will nothingness than not will.
In saying that the ascetic priest serves as a sick doctor to these sick masses, Nietzsche is suggesting that the ascetic priest directs and channels their degenerate wills to power. The three channels offered here are an extinction of the will, hard work, and the consciousness of sin and guilt.
The extinction of the will is mostly associated with Indian philosophy, and has come to the West largely thanks to Schopenhauer. The Hindu ideal of reuniting with Brahman and the Buddhist ideal of Nirvana both laud the extinction of the ego and the total disappearance of the distinct self into a greater whole. The water drop, so to speak, ceases to see itself as a water drop and comes to see itself as a part of the ocean. This is the paradigmatic example of choosing to "will nothingness rather than not will." The will here turns against itself, finding in itself something that it can overpower. The more it overpowers itself, the weaker it becomes.
In hard work, the will ceases to direct itself against itself, but rather at everything else. The result, however, is the same. The individual becomes lost in work and in the community of workers, becoming a part of a congregation rather than an individual. The individual will weakens by seeing itself in service of a larger community.
The self-flagellation brought about by a consciousness of sin and guilt ultimately serves only to increase the feelings of sin and guilt. Again, the individual's will is turned against itself, mortifying each thought and deed as sinful, and seeing the consequent suffering as just punishment.
In each of these cases the will is exercised, but the result is never a strengthening of the will. In each case, the will becomes "tamer," less capable of asserting itself and dominating others. Thus, the ascetic priest does nothing to "cure" the "sickness" of the "herd." But it would be a mistake to see the ascetic priest faced with the alternative of teaching the members of the herd to assert and affirm themselves as individuals and not choosing it. The priest does not have a "cure" at hand that he refuses to administer. Rather, he is faced with the alternatives of leaving the herd to allow their wills to collapse altogether or to lead them to some sort of exercise of their wills. Any assertion of the will, Nietzsche argues, is life affirming. The exercise of the will that the ascetic priest encourages in the herd is thus a life affirming opposition to the displeasure they feel at their suffering. Life is a struggle for which they are not strong enough, and the ascetic priest encourages them not to give up entirely but to find an alternative outlet for their weak wills. The result is that the herd develops bad taste and bad health, but this is still better than nothing.
It may seem confusing that Nietzsche might see the ascetic ideals of the herd as in some sense good for them. No doubt, Nietzsche despises the herd, their morality, their weakness, and their ressentiment. He even claims explicitly that ascetic ideals are harmful. However, if we recall section 13 of the first essay, Nietzsche would not blame the herd for being the way they are. It's not as if they could have been strong and have failed, making them somehow responsible for their weakness. Rather, they simply are what they are, with no sense of guilt or responsibility attached. Nietzsche's low esteem for them is thus more of a "sucks to be you" than a "you should be other than you are." The herd doesn't have the option of being strong, and so for them, ascetic ideals may be the best alternative. Nietzsche's main objection at this point is that these ascetic ideals have become so dominant that they have poisoned our entire species and harmed some healthy spirits that have no need of slave morality.