The best way I can explain visceral beliefs is that they are the somewhat undefinable or undescribable assumptions or beliefs behind our feelings, especially our intense gut level feelings. In some ways they are like a book of rules underlying our feelings or emotions that we do not know that we have. They come from our quest to best fulfill our felt needs in the surroundings we find ourselves in. As such they are determined by our innate qualities and tendencies (nature) as well as our environment (nuture), and which factor has more greatly determined our own visceral beliefs may vary from person to person and from visceral belief to visceral belief.
Many of our visceral beliefs are fully formed and in place before we reach the age of reason or use our own thinking or cognitive ability to consider how we should act. It is not really important to determine exactly what age this is, but it is around the age we stop believing in Santa Claus and similar things. Visceral beliefs are constantly changing slightly to incorporate all of our life experiences, but for most people they stay relatively constant throughout life. Sometimes they change drastically on the negative side with traumatic events or on the positive side with religious or spiritual experiences that leads to a very different outlook on life or sometimes with the love for a partner, child, etc, but most commonly they change slowly and grudgingly without intense guided effort.
Visceral beliefs are in some ways primitive or animalistic. This does not mean they are something to be avoided, suppressed or matured past, and that is not really possible anyway. Visceral beliefs can be modified or even replaced, but in their old or modified form they will always be with us and determine how we feel in a particular situation.