Albert Barnes Commentary Matthew 23:18

"And, Whosoever shall swear by the altar, it is nothing; but whosoever shall swear by the gift that is upon it, he is a debtor." — Matthew 23:18 (ASV)

The altar. The altar of burnt offerings, in the court of the priests. See Barnes on Matthew 21:12.

It was made of brass, about thirty feet in length and breadth, and fifteen feet in height (2 Chronicles 4:1). On this altar were offered all the beasts and bloody offerings of the temple.

The gift that is upon it. The gift or offering made to God, so called because it was devoted or given to him. The gift upon this altar was always beasts and birds.