How can I make over a church pew for outdoor use?

You could paint it with an exterior paint like you use on house exterior or doors. If you are wanting to keep natural, multiple layers of a weatherproofing poly.

Outside, uncovered, it won’t survive the water pooling on the seat, no matter how many coats of Marine Varnish one puts on it.

If it’s going to be on a covered porch, then it will Ok to leave it out for occasions.

The majority of pews are oak and that’s not an exterior-grade of wood. Redwood & cedar are, as well as some grades of mahogany and teak.

I would lightly sand it, clean it and saturate it with two thin coats of diluted Marine Varnish, working it in every pore & crack. This is UV stable exterior grade oil-based polyurethane.

Then after that has thoroughly dried 24 hours between coats, I would apply 2 more full coats, undiluted.

If you are going to poly it with an exterior varnish or an exterior house paint you will have to varnish or paint the undersides as well if you want it to last outside. I built and painted some pine tables with exterior house paint and they are outside all summer in the sun and rain. They still look good after about 7 years. I just wash them off with a damp rag and a bucket of warm soapy water with dish soap in it. If this was my church pew I would only keep it in a covered area or inside! If you try to buy a church pew now a days they can get very expensive and hard to find! I have a church pew I bought years ago for 25.00--it gets special treatment and I would never put it outside except for when I need seating for special occasions.

Sand and spar urethane finish.

marine varnish is the best I have heard about. But like some have said, I wouldn't leave it uncovered.