Jamieson, Fausset and Brown on Exodus 24:8

8. Moses took the blood, and sprinkled it on the people--probably on the twelve pillars, as representing the people (also the book, He 9:19), and the act was accompanied by a public proclamation of its import. It was setting their seal to the covenant (compare 1Co 11:25). It must have been a deeply impressive, as well as instructive scene, for it taught the Israelites that the covenant was made with them only through the sprinkling of blood--that the divine acceptance of themselves and services, was only by virtue of an atoning sacrifice, and that even the blessings of the national covenant were promised and secured to them only through grace. The ceremonial, however, had a further and higher significance, as is shown by the apostle (see as above).