How will a non-veg diet affect one's spiritual journey?
A vegetarian diet is not absolutely essential on the spiritual path, but it is certainly helpful. Quite apart from the issue of entailing cruelty to a relatively high life form, eating animal flesh tends to agitate the nervous system, thus making it harder to meditate deeply. This is due in part to the difficulty that the human body has in digesting meat, for as Swami Sri Yukteswar (Paramhansa Yogananda’s guru) wrote in The Holy Science, meat is not a natural part of human diet, as the body is not designed to digest it efficiently. It is also due to the toxins that remain in animal flesh after the animal is killed: the adrenaline that floods the animal’s body at the time of killing, and the fear that permeates the body at that time. There can also be basic health risks for the person who eats animal flesh, due to the many chemical and antibiotics that are fed to the animals to make them bigger and thus produce more meat.
Paramhansa Yogananda noted that some people do need to eat some animal flesh now and then, for health reasons. In that case, he recommended fish, chicken, or lamb, but definitely not beef, pork, or veal.