Desert-island theology is when you try to reduce a theological question down to its basic elements. This can be a really helpful activity for thinking clearly, however it need to be taken back into context. Some examples:
What is marriage? Well, if a single guy and single girl found themselves stranded on a desert island, couldn’t they make promises to Other and get married and have sex and it not be sin? Sure. At its most basic level marriage is two people in a covenanted relationship. But that doesn’t mean you and your girlfriend are married when you promise to live each other and decide to have sex. You’re not on a desert island!! The basic principles are not the only ones.
What about church? If two people find themselves on a desert island, and they read the bible and pray – yeah, it’s their church! But when you refuse to be part of a local group that has authorities and support, you’re ignoring Jesus’ people.
Desert island theology is a helpful tool for identifying important elements of ideas, but those ideas should never be left on the island.