1) Roman Catholicism is Christianity.
2) The US was founded by Christians and deists, but as a secular society. See the 1st Amendment. The Age of Enlightenment made great advances towards realizing the need for secular governments.
3) The Declaration of Independence makes not mention of Jesus, only a creator. Lots of religions believe in the concept of a creator.
4) A lot of justification for slavery, harming/killing/displacing the natives, and mistreatment of minorities were made credible (at the time) because of the Bible. It provided justification in the eyes of those doing it.
5) The US for a long time had a very small minority US Catholic population. This day, it's roughly 50% protestant and 22% Catholic.
So no, it does not appear that you know much about this friend.
#1 No it is not. In Christianity you can go straight to Jesus. In the RCC you have to go to a Priest and confess your sins, you believe the Pope is God on earth, Mary interceedes prayers, you have many Saints to pray to and there are many others.
#3 The land America got was declared under the Treaty of Paris (which was signed to end the Rev War) in which the first ten words are "In the name of the most holy and undivided Trinity" (a Christian Doctrine). President Washington In NYC concentrated America to God which he said the following statements:
A.)"We ought to be no less persuaded that the propitious smiles of Heaven can never be expected on a nation that disregards the eternal rules of order and right which Heaven itself has ordained;"
B.)"since He has been pleased to favor the American people with opportunities for deliberating in perfect tranquillity, and dispositions for deciding with unparalleled unanimity on a form of government for the security of their union and the advancement of their happiness, so His divine blessings may be equally conspicuous in the enlarged views, the temperate consultations and the wise measures on which the success of this Government must depend."
C.)"It is impossible to rightly govern a nation without God and the Bible." President George Washington
D.)President Washington was given the oath of office in New York City in St. Paul's Chapel, New York City. "This is the place where the new nation was committed to the almighty; this is America's ground of consecration"
E.) People have taken one verse out of the Bible and used it to justify whatever they want. In actual contex the Bible doesn't have the word slaves it says servants and it instructs how well to treat them.
Exodus Chap 21 verse 1 says If thou buy an Hebrew servant, six years he shal serve: and in the seventh he shall go out free for nothing. Verse 12 says He that smiteth a man, so that he die, shall be surely put to death. Servants were contracts back then. Not the lifetime slaves that America had.
F.) There are more Catholics in America than Protestants but even they are a division of Catholics as they followed Martin Luther who was a Catholic Priest that said people do not need to go through the pope to get to God. I am a Christian. I try to follow Jesus and what he taught directly. The more denominations you have the more HUMAN rules are added.
Even the Pope recently said it is dangerous to have a personal relationship with Jesus. A Christian would never believe that.