Actually, 5th Ave is a better example. I did that every year for 30 years. That proves that the businesses there think they can draw more customers by decorating their windows at Christmas. I love it and also the Rockefeller Center Christmas tree but that says nothing about this being a Christian nation. How do you define a Christian nation? My definition in worldly terms would be a nation supporting one faith is one which declares it to be one in its law like England used to do under the Anglican Church and Iran and Iraq under Islam.
The First Amendment declares America to have religious freedom, meaning that everyone's faith is allowable under the law. It doesn't say that only the Christian faith is recognized.
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Freedom of religion is a principle that supports the freedom of an individual or community, in public or private, to manifest religion or
belief in teaching, practice,
worship, and observance without government influence or intervention. It also includes the freedom to change one's religion or belief.
[1]
Freedom of religion is considered by many people and most of the nations to be a
fundamental human right.
[2][3] In a country with a
state religion, freedom of religion is generally considered to mean that the government permits religious practices of other sects besides the state religion, and does not
persecute believers in other faiths. Freedom of belief is different. It allows the right to believe what a person, group or religion wishes, but it does not necessarily allow the right to practice the religion or belief openly and outwardly in a public manner.
Freedom of religion - Wikipedia
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In India, a Hindu converting to Christianity is considered an "untouchable" and his life might be in danger. You can make a similar statement about China, Iran, etc. This is not true legally for a Christian converting to Islam in America although the reality is those who do not accept the Constitution might take it upon themselves to persecute that person. Most often this is the Religious Right.
God looks at the hearts of individuals and does not judge entire nations under the Covenant of Grace.