There are things about which you can't doubt: The whole is greater than the part, for example. And these form the bases for building certainties on that sure basis.
You might in a classroom act as if nothing is real but if a gun is pointed at you , you duck.
Also, you can do what lawyers sometimes do: Stipulate. IF this is taken to be true then the following must be true. So if you stipulate that only God can raise someone from the dead and someone says so-and-do was raised from the dead...
This is a way of separating out what you are interested in from what someone might be quibbling about in order to not answer your question. Suppose you are dealing with someone who claims not to have murdered someone you could still ask, Do you agree that IF someone did murder someone else they should receive punishment X? This will often tip you off to a lie. A person who DID murder someone will not want to even stipulate that it deserved such-and-such punishment