Verified answer

Due to the incompleteness of Tolkien's Elven languages it would be safer to go with a simple transcription of the English words in Tolkien's Tengwar (Elven script).

The only attested verb meaning "believe" would be Quenya (High-Elven) sav-. But that means "believe (that statements, reports, traditions, etc. are) true, accept as fact”, so it may not be such a good choice for this phrase.

You could rephrase however:

"á harya astar intyen"

(have faith for yourself)

Vocabulary:

á - imperative particle; followed by infinitive

harya- - possess, have; inf. harya

astar - faith, loyalty (not belief)

intye - yourself

-n - for, to; dative case ending

Dictionaries:

http://folk.uib.no/hnohf/Quettaparma.pdf

http://www.ambar-eldaron.com/telechargements/queny...

Grammar: http://folk.uib.no/hnohf/quenya.htm

For the verb conjugation see:

http://www.ambar-eldaron.com/telechargements/Queny...

Both the English phrase and the Quenya translation written in Tengwar:

http://www.flickr.com/photos/32039090@N02/82766042...

You can check the letters of the English words with:

http://at.mansbjorkman.net/teng_general_english.ht...

or http://www.acondia.com/fonts/tengwar/info/english....

For Quenya see:

http://at.mansbjorkman.net/teng_quenya.htm

or http://www.acondia.com/fonts/tengwar/info/quenya.h...

If you have any questions about this, feel free to send me a message.