This refers to the way that the people who were walking by Jesus were mocking him.
# were mocking him with each other
"were saying mocking things about Jesus among themselves"
# Let the Christ, the King of Israel, come down
The leaders did not believe that Jesus is the Christ, the King of Israel. AT: "He calls himself the Christ and the King of Israel. So let him come down" or "If he is really the Christ and the King of Israel, he should come down" (See: [[rc://en/ta/man/translate/figs-irony]])
# believe
The means to believe in Jesus. AT: "believe in him" (See: [[rc://en/ta/man/translate/figs-explicit]])