"At the beginning" or "in the beginning"?

Are both expressions

• "At the beginning"
• "In the beginning"

valid and equivalent? The first "seems wrong" to me, but it has more Google results.

They are valid but not interchangeable. I think the most important difference is that "in the beginning" seems to be an expression describing a whole period of time, while "at the beginning" more literally describes a single moment in time, similar to the difference between saying "in the morning" and "at 8 a.m."

Compare your question to "in the end" versus "at the end." "In the end" is an idiom synonymous with "ultimately." There's a clear distinction. I think the same can be said for "in the beginning"/"at the beginning."

"In the beginning" are the three words that open the Book of Genesis in the Bible. For Christians, the phrase conveys that additional sense of an origination.

"At the beginning" by itself just sounds incomplete to my ear. At the beginning of what?