Genesis 17: 7
And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee.
An oath means you vow to keep a promise.
God made an oath with Abram in (Genesis 17) when he told him at (v2) that he would make a covenant between himself and Abram, and Abram’s family would multiply exceedingly. God told Abram his name would no longer be Abram, but Abraham because he would be father of many nations. In (v6) he told him he would make nations from his seed and kings would come from him.
(And by-the-way this perfect oath affects all who obey even today. Acts 11:18 -When they heard these things, they held their peace, and glorified God, saying, Then hath God also to the Gentiles granted repentance unto life.)
At (v7) God made an everlasting oath between Abraham and himself and Abrahams seed after him. When Abraham was too old to travel he asked his servant to make an oath that he would go back to the land of God’s people and get his son Isaac a wife. (Genesis 24:1-9)
And if the woman will not be willing to follow thee, then thou shalt be clear from this my oath: only bring not my son thither again.
From Abraham’s seed did come kings such as David who was promised this same oath the prophet Samuel spoke about at (2 Samuel 7:12-17). Therefore it can be fully stated at Acts 2 when the Holy Spirit came upon many that were present and all understood the word of God in their own language:
Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne;
Christ was made a king, a priest with an oath from God. Not like the priest from Moses day of the Aaronic priesthood, but an everlasting priest.
(For those priests were made without an oath; but this with an oath by him that said unto him, The Lord sware and will not repent, Thou art a priest for ever after the order of Melchisedec:)