“For verily I say unto you, That whosoever shall say unto this mountain, Be thou removed, and be thou cast into the sea; and shall not doubt in his heart, but shall believe that those things which he saith shall come to pass; he shall have whatsoever he saith.”
Truly I tell you, if anyone says to this mountain, go, throw yourself into the sea, and does not doubt in their heart but believes that what they say will happen, it will be done for them — the mountain-into-the-sea saying uses the local geography of the Mount of Olives standing above the Dead Sea as the extreme example of faith's potential. The saying is not a formula for getting things by saying them but a communication of the scope of what faith-saturated prayer can accomplish. Does not doubt in their heart is the key condition: the doubt that undermines prayer is not intellectual questioning but the heart's fundamental uncertainty about whether God hears and acts.
Church Fathers on Mark 11:23
The Lord, leaving darkness behind Him in the hearts of the Jews, went out, as the sun, from that city to another which is well-disposed and obedient. And this is what is meant, when it is said, 'And when even was come, He went out of the city.'
The greatness of the miracle appears in the drying up so juicy and green a tree. But though Matthew says that the fig tree was at once dried up, and that the disciples on seeing it wondered, there is no reason for perplexity, though Mark now says, that the disciples saw the fig tree dried up on the morrow; for what Matthew says must be understood to mean that they did not see it at once, but on the next day.
The meaning is not that it dried up at the time, when they saw it, but immediately after the word of the Lord; for they saw it, not beginning to dry up, but completely dried up; and they thus understood that it had withered immediately after our Lord spoke.