Jesus expresses the same idea both positively and negatively to emphasize that this faith must be genuine. Alternate translation: "if you truly believe" (See: [[rc://en/ta/man/jit/figs-doublet]])
You can translate this direct quotation as an indirect quotation. This can also be stated in active form. Alternate translation: "you will even be able to tell this mountain to get up and throw itself into the sea," (See: [[rc://en/ta/man/jit/figs-quotations]] and [[rc://en/ta/man/jit/figs-activepassive]])