Well for hundreds of years the majority of Americans were white and Christians, but the entire nation has certainly never been entirely moral and decent. So don't look at the past through rose colored glasses too much.
With that said, when America was mostly white Christian Americans the country was much better off. We shot up to a world super power in record time, we didn't have mass shooters, we didn't have a divided nation, we didn't have rampant crime like we do now, we didn't have rampant drug problems, we didn't have wide spread homeless, we didn't have unsecure borders, we didn't have Americans at war with each other over other countries or foreign religions, and so on. There is no denying America was better for hundreds of years without those things.
What did have is the vast majority of Americans had American morals, values pride and standards. We had a more unified nation.