I read this quite a while ago and the book was old even when I first encountered it. I probably read it in the 1970s, and it likely dated from the late 1930s, the 1940s, or perhaps the early 1950s.
It had an odd style, a little stilted, and was fairly long, especially for the time. I read it in hard cover form, and it was at least 500 pages.
The protagonist had lived and died in our times and then been reincarnated thousands of years later. I think he was a gift, perhaps for a wedding. It was written in the first person. I'm not sure he ever mentioned his name.
Much of the story details his reaction to the future, and then how his reincarnation challenges everyone's preconceptions. It was a slow read, but a fascinating book.
At one point there were children somehow swimming with the planets. I'm not sure if I would call it science fiction or fantasy.
That's as much as I recall right now. If I think of anything else, I'll update this post.