1 Kings 1:29-30

1 Kings 1:29-30King James Version (KJV)
29 And the king sware, and said, As the Lord liveth, that hath redeemed my soul out of all distress,
30 Even as I sware unto thee by the Lord God of Israel, saying, Assuredly Solomon thy son shall reign after me, and he shall sit upon my throne in my stead; even so will I certainly do this day.
An American English Bible
With Old Testament based on the Greek Septuagint

29 Then the king swore [an oath to her], saying: 'As Jehovah (who saved my life from all danger) lives, 30 and as I swore to you by Jehovah the God of Israel, saying, Your son Solomon will reign after me and sit on my throne after me, I'm saying the same thing today.'

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29 David had been thru a lot been serving the Lord and then later following his wants and desires which lead him away from God and then he turned back to God. God had allowed him to kill Goliath and others in His name. He became king because the Lord chose him but this was no easy road, he had to wait and fight to become king. In this vow David may have though about all this and knew this nothing greater that he could make a vow on than the Lord.

30 Jesus says that we should not make vows. David had already made this promise but it without him doing was about not to happen de the kingdom being taken by another one of his sons. Solomon was not only the David had chosen but was also the man God had picked to be the next king, the prophet of God, Nathan had sent her in there to speak to him about Solomon being king.