Seitan and satan are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Seitan is a noun. It is a whet gluten based product eaten as a protein source. In a recipe, it could be used to replace or substitute for ground beef.
Satan is a proper noun. Satan is a deity that represents evil incarnate in Judeo-Christian beliefs. Sometimes Satan is a pictured as a two-horned man, sometimes he is depicted as a goat walking upright on its hind legs, sometimes he takes human form (in film and TV) as a person who is irresistible and dangerous in every imaginable way.
The following story uses both words correctly:
Schuyler was a vegetarian. At the church pot luck, he threw older church members for a loop when he said his burgers were made with seitan. They heard “Satan” and were immediately turned off.
On an ironic note, the Devil’s Food cake was eaten to the last crumb and the Angel Food cake remained untouched.