We read in John's gospel that Jesus "perceived that they would come and take Him by force, to make Him a king" (John 6:15 KJV). I fear that this is what many professing Christians desire to do today as well. They seem to believe that somehow Jesus wants to reign and rule in the affairs of the United States through political means. If somehow we can get a "conservative" president, the right mix of conservatives in congress, a conservative bench, then righteousness will reign in this country, Jesus will be king of our nation.
I find it interesting that Jesus, when He perceived the intentions of the people, departed from them. He had no interest, no desire to rule an earthly kingdom. Imagine the good He could have accomplished in His lifetime for the nation of Israel had He actually displaced the Roman empire and ruled from Jerusalem. However, that was not His true purpose in coming to earth.
We read of Him standing before Pilate, when He states, "My kingdom is not of this world: if My kingdom were of this world, then would My servants fight, that I should not be delivered to the Jews: but now is My kingdom not from hence" (John 18:36 KJV).
Why then do we insist upon embroiling ourselves in the politics of this world, when instead, we should be living for the advancement of the true kingdom of Jesus Christ. There are indeed two kingdoms, the kingdom of this world, and the kingdom of God. Let the world take care of the world, while we put our effort and energy into Christ's kingdom, where our allegiance truly belongs.
Someday the kingdoms of this world will become the kingdoms of Christ, and He shall reign forever and ever. (Rev 11:15) Until then however, this world will wax worse and worse. We as followers of Jesus Christ, can accomplish far more good through the spreading of the gospel and fervent prayer. It has never worked to legislate righteousness through political means, and it never will. The "legalizing" of Christianity during the reign of Constantine is the perfect example of the error of mixing Church and state. Let the Church be the Church, living as strangers and aliens in this present world, looking for the city which is to come (Heb 13:14), while calling people out of the darkness of this world, into the kingdom of Christ (Col 1:13).
See also this link