Was Arnold the devil or not?
I honestly don't think Oates was going for any religious metaphorical characterization here. There simply isn't any other symbolism or language to support the idea of a specific representation of the devil here. The closest I'd go to that line of thinking would be to say that Arnold may in fact represent a loss of innocence for Connie, and feels like a sort of composite character who summarizes many qualities that might disappoint a young girl on the verge of losing her virginity. He's older than he seems at first, he's not attractive, he wears false heels to appear taller, he seems to target young girls by hanging around in places where he'll find them. Despite his undesirable qualities, Connie submits to him, perhaps because she understands she will need to be deflowered at some point and it might as well be with him.
It is also possible that Arnold means to murder Connie and her stepping off the porch and walking to his car is the last part of her journey towards an immoral, decadent future. Loss of innocence can be seen as a symbolic death. This also relates to the meaning of the title, from a Bob Dylan song, which suggests a cyclical progression, an eternal return, a life cycle that will continue to happen in the same way.