“He therefore who swears by the altar, swears by it, and by all things that are on it. And he who swears by the temple, swears by it, and by him who dwells in it. And he who swears by the heaven, swears by the throne of God, and by him who sits on it.”
So now He tries to turn their thoughts Heavenward. Note the advance in thought. First the altar where propitiation can be made and men can approach God, then the Temple from which worship and prayer and incense is offered and where God can be seen as symbolically present, then Heaven where God is present in majesty, and then especially ‘the throne of God' where, as it were, God Himself is seated in glory. That was where their worship should have led them, rather than simply to admiring and concentrating on their own gifts (compare 1 Kings 8:27). Note the parallels with Matthew 5:34-35, but here the thought is not on whether oaths are acceptable, but on the fact that their attitude to oaths indicates that the whole direction of their thinking is wrong. It is concentrated on the works and contributions of man rather than on the grace and holiness of God.
Note that in Matthew 3:5 that believers will ‘inherit the earth' (receive God's fullness of blessing) but these miss out on God's fullness of blessing because their concentration is on their own giving and not on Him.