Jesus just finished turning the temple upside-down because the Jewish elite where preventing Gentiles from worshiping God because their merchandise was in the only place they could congregate. Also, they made a mockery of God's passover. Now the Jews looked for a way to kill Jesus.
Meanwhile, the tree Jesus cursed had died. He used that opportunity to tell the disciples to have faith in God. If they have faith as small as a mustard seed, they can move mountains.
People use the phrase "moving mountains" to mean any obstacle in their lives. Do I have an amazing shyness that prevents me from asking people to do stuff? Maybe that's a mountain that my faith can move.
I agree, faith can move it, but faith in what? My faith should not be in me or my willpower. It will fail every time. My faith should be in the Lord Jesus only. Only he can change my heart and make me bold. Only Jesus can take me through situations I'd rather not be in. I have no power on my own except to rebel against God and do what I want.
This selfish power is what the Jewish leaders had. They had too much self-esteem and did not notice when God the Son came to the temple and turned over tables. Self-love and self-faith will only seek to kill Jesus and get rid of God. True faith will sit and watch while God does all the amazing work in life.