Nemo potest dare quod non habet is a Latin phrase that translates “No one can give what he has not.” It is a cliché because it is so very true.
If you do not have an apple, you cannot give someone an apple.
If you do not have an umbrella, you cannot give someone temporary shelter from rain.
If you do not have sound catechesis, you cannot pass on the Catholic faith to anyone else.
If you do not know the faith, you cannot evangelize. You can only confuse and lead someone to error and damnation.