Persistence in doing something despite difficulty or delay in achieving success (Oxford Dictionary).
Jesus said: 'Ask, and you will receive. Search, and you will find. Knock, and the door will be opened for you'. (Matthew 7:7).
Don't give up
If at first you don't succeed, try, try and try again
“It does not matter how slowly you go as long as you do not stop.” ¯ Confucius
“Never, never, never give in!” ¯ Winston S. Churchill
“It always seems impossible until it's done.” ¯ Nelson Mandela