I recently read the book “Contact” by Carl Sagan. You may remember the brilliant but oft criticized Zemeckis movie with Jodie Foster and McConaughey.
I’m not going to get into a breakdown of either work, but if you want to watch a movie (or read a book) that attempts to deal with and reconcile science, religion and faith and what it means to be human as well as 2001, or Close Encounters, for sure check out “Contact”.
Anyway, in the book, aliens make contact with Earth through a radio signal sent from Vega. It is discovered and decoded and how it is known to be generated by intelligent life, as opposed to random noise, is that they are transmitting prime numbers.