Good Christians differ here on earth about the exact meaning of this. I’m of the following that St. Paul is talking about the creation of a new tree out of both Jew and Gentile, which is called the Christian church. “We’ll all understand it better bye and bye.”
Another hitch in a direct carry over is that America as a whole was never officially Christian like Israel being officially Jewish, although for years many individual states endorsed particular Christian denominations (with ugly, cruel, un-Christian results sometimes). So it’s quite possible for Christians in America to fast and pray their hearts out and they will only end up getting personally blessed with the country still going to hell in a handcart. This is a necessary warning, I believe, so that Christians do not hitch their wagons to the wrong star, so to speak.
It ALL depends on Gods plans.
I believe we are in the days leading to the end times. That we are at the verge of the tribulation period.
My prayers are that God would cause a revival in this nation and restore us to where we once were. But, when I look at Bible prophecy, I don't see a nation like the US in it. It doesn't mean we aren't there, but I'm not sure the nation of Ronald Reagan would stand by while Israel is attacked, and yet the Bible tells us nobody comes to Israels aid, and that God alone protects her.
If Gods plans are to humble this nation, so be it. I'll still pray. Maybe prayer will move Gods hand to move the hearts of many men towards him. Drawing unbelievers to him and believers closer. May the difference between believers and unbelievers in this nation be so great, that we expose the folly of unbelievers for what they are.