Confidence is part, I'm sure trust is another.
And not only confidence in one's ability to throw, but also of one's knowledge - you need to know the O cold, before you start throwing guys open. If you KNOW that every time on this route Sammy is going to take 10 steps, cut right and be open you can learn to trust that he'll be there and make the throw before the WR breaks.
And that's where you have to trust that the receiver is going to do what they're supposed to. If the system is too complicated, or the players too dumb and they keep messing up on practice, it's going to take a lot to get that QB to throw the ball before the guy is coming out of breaks...
Regardless, as has been stated, we'll see what Tyrod learned soon enough!