Didn't want to get off topic in the "Obscure Fandoms" thread haha.
The quote is from William Shakespeare's "Hamlet". It's when Polonius' son, Laertes, is going to university. He's trying to give his son advice for being on his own which contradicts itself at times (speak your mind, but not too much, etc). The last piece of advice he gives him (I.III.78) is, in it's full quote, "This above all: to thine own self be true. And it must follow, as the night the day. Thou canst not then be false to any man. Farewell. My blessing season this in thee." It's Polonius reminding his son that above all the advice he has been given, the most important thing is to be exactly who he is.
My entire account here and everywhere is based around Shakespeare and this is a line that has stuck with me through all my readings and college essays. It's one of the most simple yet brilliant lines in any of his plays that I think we can all relate to. Just be who you are.
Wanted to add in that while Hamlet itself is not necessarily a religious text, there are many literary scholars that believe Catholicism plays a huge part in the play. And there are many parallels and references to the religion as a driving force for Hamlet's motivations (whether or not he should kill Claudius, what happens after we die, and so on).