John Gill Commentary Matthew 23:20

"He therefore that sweareth by the altar, sweareth by it, and by all things thereon." — Matthew 23:20 (ASV)

Whosoever therefore shall swear by the altar,
&c.] Not that Christ allowed of swearing by the altar, or by the temple, or by heaven, or by any creature, animate or inanimate; for such swearing is elsewhere disapproved of by him, and forbid, but if a man did swear by the altar, he ought to know, and consider that he not only sweareth by it ,
but by all the gifts, and offerings that are brought, and laid upon it, and by all things thereon ;
whatever gifts and sacrifices are offered upon it; which, by being put there, become holy, as the altar itself: so that he that swears by the altar, swears also by the gifts of the altar; and consequently, according to their own traditions, such oaths must be binding.