Mark 11:23

For assuredly, I say to you, whoever says to this mountain, “Be removed and be cast into the sea,” and does not doubt in his heart, but believes that those things he says will be done, he will have whatever he says.

Do you notice that this scripture says doubt with the heart?  Which means we have to believe with our hearts and not with our minds.  We need to be careful and make sure we are not just agreeing with a concept in our minds but fully persuaded in our hearts.  To believe in something that strongly we need to meditate on the scripture about what we are praying for until we build the faith.  Build the foundation of faith in the heart and then move the mountain.