So Jesus answered and said to them, "Assuredly, I say to you, if you have faith and do not doubt, you will not only do what was done to the fig tree, but also if you say to this mountain, 'Be removed and be cast into the sea,' it will be done. And whatever things you ask in prayer, BELIEVING, you will receive."
Jesus is laying out one of the biggest concepts ever for his disciples, one that is required for salvation: Faith. "If you have faith", that is, if you have not buried it or lost it or succumbed to doubt, then this passage applies. "And do not doubt"...James says that a person who doubts is like a person tossed around at sea, the waves knocking him around and beating him up. Doubt is the devil's work, never to be trusted.
This is King Jesus who is decreeing this statement, and what He says is law.
An executive order, if you will.
"Whatever you ask in prayer, believing, you will receive." Prayer needs to be infused with faith, and when we ask for anything, we need to believe it's possible. That God cares. And listens. And acts on our behalf, in our best interests. Believe that God is who He says He is and will do what He promised. He is King, and He can do anything. Just ask in faith, and do not doubt.