Charles Ellicott Commentary Numbers 19:10

"And he that gathereth the ashes of the heifer shall wash his clothes, and be unclean until the even: and it shall be unto the children of Israel, and unto the stranger that sojourneth among them, for a statute for ever." — Numbers 19:10 (ASV)

And it shall be unto the children of Israel, and unto the stranger ... — So the promise of the remission of sins through Christ Jesus was not only to the Jews and to their children, but also to all that were afar off. (See Acts 2:39.)