You're going to get a different answer depending on who you ask because people will probably interpret the Founding Fathers' words to purvey their own beliefs. One of the most blatant statements that the USA was not founded as a Christian nation is evident in Article 11 of the Treaty of Tripoli.

the government of the United States of America is not in any sense founded on the Christian Religion

John Adams was heavily involved with the Treaty of Tripoli. Although it was officially Arabic, the English version of it was ratified. Critics question the translation of Article 11. So where does that leave us?

John Fea, a professor at an evangelical college, wrote a book aptly titled: Was America Founded as a Christian Nation?: A Historical Introduction. He makes several arguments against the notion that we are a Christian Nation. Arguments against the King of England, for instance, were based more in Enlightenment political theory than the Bible. Lines such as, "no taxation without representation" do not come from Christian influence. He does admit that Evangelical Protestantism shaped pre-Civil War culture in America and that many Founders believed religion was necessary to maintain a "virtuous republic." That does not mean religion = Christianity. Thomas Jefferson and Benjamin Franklin were also Deists, as were other Founders. Scholarship has debated the extent of influence for non-Christian religions like Deism and how much of a fringe theory it was actually.

Straughn & Feld (2010) assert that at the beginning of the USA, it was predominantly Christian; however, the idea of a Christian Nation is relatively new. By this, I mean that what people refer to as a "Christian Nation" aren't necessarily a reference to demographics. It is "a discursive practice that seeks to align the symbolic boundaries of national belonging with the boundaries of the dominant faith community." The ELI5 version of this is that Christians seek to call the USA a Christian Nation to justify their belief system as ingrained and historic.

So where does that leave your original question? In limbo. There is no easy, simple answer. Unquestionably, Christianity had a large following in America - we just debate on its influence on the Constitution despite barring religious tests for office and no national religion. You can easily argue that demographically America was founded as a Christian Nation. But since the framing of "Christian Nation" has more to do with historical morality and as justification for current ethics, the discussion is nuanced. Sorry, it might not be the answer you expected. You're also unlikely going to change your friend's mind. The fact that Article 11 specifically articulates "NO" and it still causes debate should indicate that this is a debate with no answer unless we resurrect the dead and ask them.

Sources:

https://avalon.law.yale.edu/18th_century/bar1796t.asp

Fea, J. (2011). Was America founded as a Christian Nation?: A historical introduction. Westminster John Knox Press.

Grasso, C. (2008). Deist monster: on religious common sense in the wake of the american revolution. Journal of American History, 95(1), 43–68. https://doi.org/10.2307/25095464

Straughn, J. B., & Feld, S. L. (2010). America as a "Christian nation"? Understanding religious boundaries of national identity in the United States. Sociology of Religion, 71(3), 280–306.

I answered a similar question a few months ago. Historian John Fea begins his book Was America Founded as a Christian Nation? by explaining "the answer to this question really depends on how we define our terms. What do we mean by terms such as 'Christian,' 'founding,' and 'nation'?"

There have been divergent opinions about whether the United States had a Christian character among the public since its founding. Legally, at the federal level the United States Constitution specifically avoided establishing any religion. However, the Constitution did not interfere with existing religious establishments that were already present at the state level. Some of the founders of the United States, such as John Adams, were in favor of state supported religion, while others, like Thomas Jefferson, vigorously opposed it.

Massachusetts became the last state to abolish its state church in 1833. The Supreme Court definitively decided that states could not support religion in 1947 with the case Everson v. Board of Education.

I'd be glad to answer any further questions you might have about this.

Suggested Reading:

John Fea. Was America Founded as a Christian Nation? A Historical Introduction (Louisville, KY: Westminster John Knox Press, 2011).

u/LordSariel discusses this here.

u/uncovered-history goes into it here.

Here is an AMA with historians Daniel L. Dreisbach and Mark David Hall about this topic.

Echoing what others have said, that very much depends on how you define it. In the simplest terms we absolutely were not started as a Christian nation. We took a series of colonies with official religions and government sponsorship payments to the church and prohibited those activities in our new government. It's amazing to me people can still argue we were started as a Christian nation with such strong actions against state religion. What we were created as was a nation that was composed of christians.

I recently answered a question about founding diests and if we started as a Christian nation that may shed some more light for your debate: https://www.reddit.com/r/AskHistorians/comments/g9z1pb/how_many_of_americas_founding_fathers_were_for/