Como dizer "Seja firme/ Tenha fé" em inglês

"Stick to your guns" is possibly better translated as "Não mude de opinião." It does not mean "Be strong" or "Be brave".

It is probably military in origin where it means "to keep fighting, not to abandone one's artillery pieces to the enemy."

THEFREEDICTIONARY

Fig. to remain firm in one's convictions; to stand up for one's rights. (Fig. on a soldier remaining in place to fire a gun even when all appears to be lost.) I'll stick to my guns on this matter. I'm sure I'm right. Bob can be persuaded to do it our way. He probably won't stick to his guns on this point.
See also: gun, stick

McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
stick to your guns (British, American & Australian informal) also stand by your guns (American informal)
to refuse to change your ideas although other people try to make you change them
Usage notes: If a soldier sticks to his guns he continues to shoot at the enemy.
David's family were against him becoming an actor but he stuck to his guns. Stand by your guns and don't let them talk you into working full time if you don't want to.
See also: gun, stick

Cambridge Idioms Dictionary, 2nd ed. Copyright © Cambridge University Press 2006. Reproduced with permission.
stick to your guns
to refuse to change your beliefs or actions My parents want me to study accounting, but I'm sticking to my guns and majoring in philosophy.
Etymology: based on the military meaning of stick to your guns (to continue shooting at an enemy although it puts you in great danger)
See also: gun, stick

Cambridge Dictionary of American Idioms Copyright © Cambridge University Press 2003. Reproduced with permission.

Complemento:

Have faith: tenha fé

''Have faith, Alexandra,'' he said. [Longman]