In the front of our church, built in the 1880's we have two Oak doors that are in need of superior weather protection. It is in the western NY snowbelt area. The wood is in GOOD condition but is in need of a quality protective covering. Varnish or shellac was used in the past. Sanding off the old varnish is needed before a natural clear coating is applied. But what is a high-quality to be used?

I like Pratt and Lambert.
The grain in oak is pretty deep and needs to be saturated so that moisture doesn't seep in. Oak turns black from water. I'd do 4 coats. Special care to the edges and end grain.
But the main concern with exterior doors is UV. There is no clear finish that protects against it. You may have to keep refinishing year after year, depending on the exposure to the sun. A light sand, a very light coat.

I doubt it is shellac, it doesn't weather well at all. Pratt and Lambert is good, Minwax Helmsman is another choice. If you have a heavy sun or weather facing door, consider going to a marina and see what they recommend for brightwork. I believe Sikkens has a marine product and they mfg. very high quality wood finishes.

Spar varnish is the best way to go.