Romans 13:1-7
13:1 Let every person be in subjection to the governing authorities. For there is no authority except from God, and those which exist are established by God. 2 Therefore he who resists authority has opposed the ordinance of God; and they who have opposed will receive condemnation upon themselves. 3 For rulers are not a cause of fear for good behavior, but for evil. Do you want to have no fear of authority? Do what is good, and you will have praise from the same; 4 for it is a minister of God to you for good. But if you do what is evil, be afraid; for it does not bear the sword for nothing; for it is a minister of God, an avenger who brings wrath upon the one who practices evil. 5 Wherefore it is necessary to be in subjection, not only because of wrath, but also for conscience' sake. 6 For because of this you also pay taxes, for rulers are servants of God, devoting themselves to this very thing. 7 Render to all what is due them: tax to whom tax is due; custom to whom custom; fear to whom fear; honor to whom honor. NAS
Although the site you linked to is correct is stating the United States is not a Christian country per our Constitution, your interpretation of the 1st Amendment exposes your ignorance of the early history of this country. The 1st Amendment's purpose was to avoid the religious problems that plagued Europe where Catholics persecuted non-Catholics, non-Catholics persecuted Catholics, the Church of England persecuted other denominations and etc. We see the same sectarianism in Iraq where Sunni's and Shiites are fighting for religious supremacy.