What material should you use for the walls of a bunker?

Rapture is a heresy. One must hide within a small group (10-12 people according to saint Seraphim of Sarov from Russia, 10-15 people group according to saint Gabriel Urgebadze from the country of Georgia) in order to escape the unforgivable mark of the beast. No documents; documents are from Satan; burn them (even documents of deceased relatives). No electronics once you hid so that you won't be tracked (even an old broken unplugged TV set from 1970's will show the infamous evil flying antichrist using Tesla's ether). [Saint Vyacheslav Krasheninnikov] Mark of the beast is given with World Passport (grey plastic card with no name on it); antichrist will release prisoners / insane asylum people to help him mark everyone. Police will mark people on highways; food stores will mark people who steal marked food (because marked food can only be swallowed by a marked person who has three strands of DNA instead of two). Reject vaccines, temperature scans, COVID tests (or similar), etc.

Use reinforced concrete or self-healing concrete for greater results. Place metal beams across the trench to support your bunker against collapsing. You can also use reinforced concrete to create an extra outer layer for added protection. The walls of your bunker also need to be at least 1-3 feet thick.

My father's bunker was 18 inches of steel-reinforced concrete and a 4-inch door.  Mine has a double wall of granite blocks, each 28 inches thick with two feet of dead air between the walls.   It came with the house.

You have to pay attention to ground water and remember that regular concrete is porous.

Steel reinforced concrete

Lol bunkers and bunker complexes are all steel and concrete....a nice fantasy but we all know you won't be building one. Interesting question though 👍👌