Knowing our purpose is fundamental, however we also need to have belief we can achieve it, along with the motivation to execute.
Think of a mountain as your purpose.
Before you attempt to walk/climb the mountain you have to have some belief that you will succeed - otherwise why attempt in the first place.
Then we need the will and motivation to actually place one foot in front of the other.
The belief is the fuel to the motivation to execute.
Go after your purpose today!