Peoples wrong opinions don't interest me. I'll simply be going but the definitions of the two words and the fact is we are not. We are simply a country with a majority of christians in it like myself. By definition USA has never been a christian nation and it isn't now.
How about opinions based on actual fact? The first settlers in the New World came here in search of religious liberty that they were unable to obtain under the authority of the Archbishop of Canterbury and state ordered religion in England or the Catholic pope who held authority in most of western Europe.
At least nine of the original 13 colonies were established via specific established churches/Christian sects/denominations and required office holders to be confessed Christians loyal to those sects/denominations. A belief in Jesus Christ was mandatory.
(Subsequently 49 of the 50 states address their constitutional preambles to God/Creator or a similar term and the 50th makes reference to God in the body of its constitution.)
John Locke's two treatises of government were profoundly influential on the early culture of America and the Constitution it ultimately developed, and these were based on the concept of God given/natural law that is the very foundation of what the Constitution was intended to protect.
The Founders themselves were almost to a man devout Christians and even the very few 'deists' among them embraced and endorsed Christian principles, the Bible upon which Christian principles are based, and the ethics and morality promoted by the Christian religion.
Ultimately they gave us a 'secular' government so that no predominant Christian group or any other religion would be able to lord it over any others--no monarch, pope, or archbishop would have authority over the laws and governance of the people who would themselves ordain and establish a government that would allow them to govern themselves.
But it was a government based on Christian values established by Christians for a nation of almost all Christian citizens at the time.
We were and are a Christian nation, and because we are, people of all faiths have found equality, security, and peace here.