Which is the correct spelling - uwierzyć or wierzyć?

a friend of mine is getting a tattoo saying "uwierzyć". i always thought it was "wierzyć". sorry if this is a silly question to most, but whats the difference in the meaning considering the extra U in the spelling?

wierzyć is continuus act and uwierzyć is a single act.

On wierzy = he believes
On wierzył = he believed (but now he doesnt... if he would still believe we would say "on wierzy")
On uwierzył = he started to believe (a single act that made him to believe)

You can "wierzyć w Boga" (you have always believed in God - its continuus)
and you can "uwierzyć w Boga" (you didnt believe but something happend that you started to believe - its a single act that made you to believe)

In other words:
wierzyć = to believe
uwierzyć = to start believe

thank you very much for the swift reply. :)

very, very helpful!

Uwierz w siebie = start to believe in yourself! ;)

Yeah, that's my understanding too. If I were to use that word in particular personally I'd use wierzę instead -> I believe or wiara -> faith.