Having urged them to follow Jesus all the way to shedding blood, if necessary, HW continued by prompting them to straighten up their lives. Lives follow attitudes!
A comparison of the mountains of origin helps make the choice clear. Sinai (Moses) was dark, rumbly, stormy and frightening. Zion (Mesiah) was vibrant with life and godly people--even angels; but especially with the better blood.
A shake-up was impending, and one of these two would fall. Allegiance and identity came with "not refusing Him who called from heaven" (i.e., willingly following the voice and call of Jesus).
So, these faithful followers were about to be "shaken, but not stirred"!