There is absolutely no reason to need to pretend to be Christian while visiting the US. The US doesn't have an official religion and is a very diverse nation where people travel often. It is also a very large nation, and unfortunately some people do commit crimes against people for their religion/lack of religion. This is like any other diverse nation. There's no issue with being a secular teenager in America, unless you actively argue with those who have different beliefs than yours. (And if you do actively argue, you don't typically have threats of violence or anything; its just the people of the majority belief may not want to be friends with you).
In the United States in 2009, a nation of 300+ million people had 1376 hate crimes with a religious bias; the majority being anti-Jewish (70%), anti-Islamic (9.3%). Only 0.7% were anti-atheism/agnostic. Also, while I couldn't find data that broke down by victim-group by type of crime (so this also includes hate crimes committed against other races, sexual orientations), a large majority of the hate crimes are vandalism/property-destruction/intimidation (63%) and adding in simple assault (assault without intent of injury) and other property crimes you cover (~90%) of hate crimes. The other 10% is mostly aggravated assault with the addition of about 17 (0.2%) that were murders or rapes; and many of these are possibly based on racial or sexual orientation motivation. Note the number of Hate Crimes in Great Britain is similar (1621 religion hate crimes), despite having 1/5th the population of the US.
When meeting strangers it is best not to deliberately insult/mock/convert them from their own religious/political/other beliefs--this is regardless of where you are traveling. People often get upset when their beliefs are mocked, but again this turning into assault or worse is extremely rare. However, if your waitress says "God Bless" or something and you mock her for believing in an imaginary being in the sky (rather than just ignore it), I wouldn't be surprised if you get worse service. Also, note only about ~20% of Americans actually go to church/synogogue/mosque every week, though ~40% will report weekly attendance. While politicians do pay lip service to God/Christianity in US politics more than in some other nations (e.g., European nations), its mostly playing politics. When surveyed on religion about 75% of Americans self-identify as Christian (25% Catholic, 16% Baptist, 8% Methodist, ...), about 5% are Jewish or Buddhist or Muslim or other religion, 15% reply no religion, and 5% refuse to answer.