More natural ways to say it:
* Pag naniniwala ka: This is a more informal and common way to say it.
* Kung naniniwala ka: This is a more formal way to say it.
* Sa panahong naniniwala ka: This emphasizes the time aspect of believing.
Examples:
* Kapag naniniwala ka sa Diyos, magiging masaya ka. (When you believe in God, you will be happy.)
* Kung naniniwala ka sa akin, sumama ka sa akin. (If you believe me, come with me.)
* Sa panahong naniniwala ka sa sarili mo, magagawa mo ang lahat. (When you believe in yourself, you can do anything.)
Ultimately, the best translation will depend on the specific situation and the intended meaning.