"I tell you the truth." This phrase adds emphasis to what Jesus says next.
# whoever says
"if anyone says"
# if he does not doubt in his heart but believes
Here "not doubt" is a double negative meaning "truly believe." Jesus says it both ways for emphasis. AT: "if he truly believes in his heart" (See: [[rc://en/ta/man/translate/figs-doublenegatives]] and [[rc://en/ta/man/translate/figs-parallelism]])