At what internet site can I find a description of how religion affected the life of an "average" person of the early 1800s?
I assume you mean the early 19th century, as the phrase "early 1800s" would actually refer to 1801 through 1803. There are many internet sites with information on religion in the period, and the trends in religion and other aspects. As to the effect of religion on everyday life in that time for an "average" person, most schools were religion based so almost anything on education in the era would probably mention it. I am listing several sites below, but as you do not specify what country or religion you are speaking of I will assume you mean Christianity in the United States. If you meant otherwise, please ask again. Some of these sources do deal with other religions and other areas of the world. I also suggest you try the link below to the eNotes site on religious influence in early America.
It may be important here for you to think about exactly what you mean by an average person. Older people tend to be more religious than the young, probably by virtue of being closer to death. Rural societies are often very religion-oriented, perhaps since there is less to do in a hard rural life than in town, leaving time for little more than work with church often the only social scene. The vast majority of people in America at the time were professing Chritians, although the percentage of truly committed religionists is probably debatable. Today, approximately 86% of Americans regularly attend one or another Christian church, and the percentage was probably higher then.