In terms of dynamics, you want to be subtle. No full-blown crescendos. Think not in terms of dynamics when playing Bach, but shape and structure.
As for the harpsichord, not necessarily. Some of the pieces can be interpreted as for the harpsichord, but Bach never specified. He simply said klavier. Meaning it could be a harpsichord, clavichord, organ, and whatever keyboard instrument was around at that time.
As for religious vs secular,
To Christians: The bible teaches us to "eat, drink, sing and dance in the name of the lord" (Don't remember where it's from and most certainly I didn't quote it right. But that's the general idea). While the music we play may be secular, we can still glorify God through it. It's as simple as playing for God instead of playing for yourself. And when playing for an audience, just remember that God is listening to you, too. "Bach gave us God's Word, Mozart gave us God's Laughter, Beethoven gave us God's Fire. God gave us music so that we are able to pray without words."
To non-christians: While not all of Bach's music was religious, some of his secular music can have "religious" qualities. Bach's music has a sort of a pure and holy quality to it (not sure those are the right words to use). Besides Bach was a devout Christian and it's highly possible that he felt that his talents should be used to glorify God, even in his secular music. That's how Michelangelo felt about his artistic talents.
And please let's not start a religious debate. Not that we have, but just saying, don't start one.