It really depends on whether (a) you're learning the game from scratch by yourself, (b) you're learning from scratch with a group, or (c) you're learning it to join an already active game.
(c) is the most forgiving, as long as the players and GM (if there is on) understand that you're new and work with you to remedy the situation.
(b) comes up most often. Absolutely it's impossible for someone to learn all the rules at once before playing the game, but at the very least the GM needs some amount of preliminary mastery or else the initial game sessions will be entirely comprised of everyone looking up rules (= boring and annoying).
Personally, if I'm a player I spend a while on the introduction and overview, getting a feel for how the system is supposed to work together (in the authors' eyes). Then I skim through the character generation options and pick out a few that I like, and read about their mechanics more in-depth so I could start to put together a character. I find that the character generation process teaches you a lot about how the game works.
And I wouldn't usually volunteer to run a game in a system that I hadn't read thoroughly.