I've talked before about the way that spirituality can sometimes end up affecting the way we live our lives in a blog called 'Reshaped Living' and I think it's important to understand the way that personal gnosis can directly influence us. Another way that spirituality can influence our lives is with the taking on of spiritual prohibitions or directives, which may be related to our spirituality in a wider sense or may be specific to us and come from an initiation or intense spiritual experience.
We see personal prohibitions or directives in many different religions including mainstream monotheistic ones, particularly around food where a religion might declare a certain food off limits for followers of that religion, or in turn might require the consumption of something. In historic Irish paganism these prohibitions and directives would be called geasa (singular geis), although it should be understood that geasa were not taken lightly. Except where they are specific to a role, like kingship, they were for life and once in place remained in place until the person died.
A geis is something the you either must do or must not do in order to maintain your luck and health, and breaking a geis means certain doom usually orchestrated by Otherworldly powers. We can find a wide array of examples of geasa in Irish mythology from those placed on kings when they took the crown to those of a more personal nature that might might be given at birth. A prohibitive example might be taken from Da Derga's Hostel were Conaire isn't supposed to invite a person alone into a place he staying in after sunset, while in contrast a directive geis would be seen with Fergus's requirement always to accept hospitality offered to him. Geasa are never, in stories, taken on by a person but are always placed on a person by an outside force or power. They also in many examples relate to an individual's spiritual connection to an animal, other being, or group; we see this in Conaire's geis not to hunt birds to whom he was related through his Otherworldly father, Cu Chulainn's not to eat dog meat since he was connected to that animal through his name, and Diarmuid's not to hunt the Otherworldly boar that his fate was bound to.
Some people argue that geasa only apply to kings, heroes, and other very rare important people based on the examples we have from mythology but I think there is a strong argument from folklore that the idea behind geasa was applied to many people across demographics in different ways. Although I might not call them geasa in the modern world the underlying concept of something that must be done or not done to maintain one's luck and health remains true. Yeats relaying an anecdote about a fairy doctor relates specific habits and dietary restrictions, such as not drinking alcohol or eating meat, which were strictly adhered to and had clear spiritual overtones. Cultural or communal prohibitions, such as not disturbing fairy mounds, also argue for a wider application of this concept.
In modern spirituality a person might acquire such a sacred prohibition when they achieve some type of initiation; for example when I became a priestess of the Othercrowd I was given a prohibition not to cut my hair. Interestingly I know several people who have a similar prohibition against hair-cutting for different spiritual reasons in paganism. Such a thing could come from the person initiating you, from the Gods in whatever form you feel such messages come, or may be a standard thing in your tradition for that type of ceremony. Becoming a priest or priestess in particular often seems to come with a sacred prohibition or prohibitions for people. As with the older concept, these prohibitions are generally permanent and cannot be transgressed without serious consequences for the person, and so should not be taken lightly or viewed as something to jump into getting.
A sacred prohibition can also come in the modern world through pure personal gnosis, although I will personally caution here that in these cases because of the gravity of these prohibitions I always recommend double or triple checking the message. This can be done by asking a neutral third party - someone who has no stake in the answer - who is good at divination or channeling to see if they get the same or a similar message. To use myself as an example again (because I don't like using other people as examples without permission) I also have a prohibition from Themselves not to enter into a Christian church or any place where active Christian worship is being conducted*; as I have no dispute with Christianity myself this prohibition surprised me and I was careful to get it verified before accepting it as genuine. People may have prohibitions through personal gnosis that could include an array of different things but the most common ones I have seen or heard of relate to food, drink, hair, or the need to always do or say something specific at certain time or place.
Sacred prohibitions in the modern world are not a subject we discuss often, nor are they an aspect of modern spirituality that is often focused on. Yet the core idea of having a spiritual prohibition or directive is not uncommon in my experience and is something that I not only have myself but also that I know many other people who have. These prohibitions or directives can impact a person's life in ways that may include social aspects, and I think for that fact alone it's worth wider understanding and consideration.
*I have my suspicion as to why this is, and I think it relates to a long standing animosity between some of the Daoine Maithe and the new religion. Emma Wilby discusses some aspects of this in her book 'Cunning Folk and Familiar Spirits' but it boils down to the way the Church tried to demonize the Good Neighbors, and the way that hostility became two-sided over time.
Yeats, W., (1888) Fairy and Folk Tales of the Irish Peasantry http://www.sacred-texts.com/neu/yeats/fip/fip43.htm