From The Unofficial Redwall FAQ:
Is there religion in Redwall?
No. Although the terms "abbey", "abbot" and "abbess" are usually associated with the Roman Catholic Church, Brian Jacques has said repeatedly that there is no religion in Redwall. He chose an abbey with an abbot or abbess instead of a castle with a king or queen because he wanted a friendly, peaceful place, not a fortress for war. The crosses or other religious imagery that may appear in some illustrations is the mistake of the artist(s). Redwall Abbey is simply a place of peace, friendship and good will, open to anyone.
From an interview on the official website:
The Redwallers live in an abbey, that means they must have a religion, what religion are the Redwallers?
There is no religion in my stories and no hidden meanings. What you see is what you get. The Abbey is just a place of peace and comradeship, where creatures choose to live together.
Why did you choose an Abbey instead of, say, a castle? Are there more of them in England?
I chose an abbey because it is a place of peace and tranquility, where ordinarily the cares of a busy and frantic world do not intrude.
Some readers have pointed out a "Saint Ninian's church" referenced in the text. If there is no religion, why would there be saints? However, Jacques apparently provided a later explanation for the seeming inconsistency:
According to a popular legend, the family of a Mossflower mouse named Ninian had grown too large for their abode, so Ninian decided to build a new home for them. However, he was so lazy that his wife performed most of the construction, prompting her to erect a sign saying "This Ain't Ninian's!". Generations of weathering eventually faded off the first three letters and the exclamation point, making the sign read "Saint Ninian's"; this reportedly caused the building, when it was rediscovered, to be mistaken for a church.
Personally, I think that the evidence seems to indicate that he started out with the framework of there being religion and then changed his mind, possibly around the same time that he started to downplay mentions of human-sized carts, milk, etc, but the Word of God (heh) from the author is that they do not worship anyone or anything because there is no religion.