For a substantial part of American history, the phase "the Christian thing to do" was synonomous with the compassionate and charitable thing to do. This persisted throughout the 1800s and well into the 1900s.
Somewhere along the line, that secondary meaning of the phase was lost. The words still have a residual familiarity, but the content has been lost. Christians lost their way and became righteous and wrathful and started to confuse right wing politics with basic human decency. Sometimes, Christians get it right. But, it has become the surprise rather than the expectation, provoking comments like: “I never thought I’d see a church do something so Christian.”
I'm not a sharp enough of an intellectual historian to tell you just when it happened, or how. But, it has happened. And, I wish I could tell you a little better how it did.