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.
Note 4 at Mr 11:23: As explained in note 4 at Mr 1:42, faith is released by speaking words. Notice that speaking is emphasized three times in this one verse, and the Lord commands us to believe that what we say will come to pass. We are to believe in the power of our words.
Failure to believe in the power of words won't keep this law of God from working. The last part of this verse says, "He shall have whatsoever he saith." If we receive this instruction and begin to speak words in faith that line up with God's Word, then we will have the positive results that follow. But if we refuse this lesson and continue to speak words of doubt, we will eventually believe them and have the negative things that these words produce.
As stated in Mt 12:36-37, every word counts. There are no such things as "idle" words that will not work for us or against us. Death or life are in the power of every word we speak (Pr 18:21). Our words can be our most powerful weapon against the devil, or they can become a snare of the devil (Pr 6:2). Faith-filled words can move mountains.
Note 5 at Mr 11:23: The physical mountain that Jesus might have meant or have been pointing to would either be the Mount of Olives or one of the mountains that Jerusalem was built on (see note 1 at Joh 5:1). However, Jesus was using the word "mountain" symbolically to refer to any high obstacle that might stand in our way.
Jesus said we should speak to the mountain, or the problem. Most people speak to God about their problem, but few follow Jesus' instructions and speak directly to the "mountain." God has put certain things under our authority, and we must exercise it. When a problem stands in our way, we must speak to the problem and command it to get out of our way in the name of Jesus.
Note 6 at Mr 11:23: The born-again spirit of man is not the part that doubts. Doubts come to the mind. Therefore, this is another confirmation that the heart, as the Scriptures use it, refers to the whole inner man: spirit and soul (see note 3 at Mt 12:34).