20 As they passed by in the morning, they saw the fig tree withered away to its roots. 21 And Peter remembered and said to him, “Rabbi, look! The fig tree that you cursed has withered.” 22 And Jesus answered them, “Have faith in God. 23 Truly, I say to you, whoever says to this mountain, ‘Be taken up and thrown into the sea,’ and does not doubt in his heart, but believes that what he says will come to pass, it will be done for him. 24 Therefore I tell you, whatever you ask in prayer, believe that you have received it, and it will be yours. (The Holy Bible: English Standard Version. (2001). (Mk 11:20–24). Wheaton: Standard Bible Society.)
23 Truly, I say to you, whoever says to this X … and does not doubt … but believes that X … it will be done for him.
~ My guess is that this is saying that faith and doubt are mutually exclusive when their object is the same.
Its like saying that you cannot believe that a rose is red and not-red at the same time.
I will have to keep thinking about this.