In Genesis 21:23, this signifies affirmation.

The Lord swearing by himself signifies that divine truth testifies, for He is divine truth itself, and this testifies from itself and means itself. It says 'Jehovah swears' because the children of Israel had a representative church, and so the Lord's conjunction with the church was represented in a covenant similar to two people swearing their trust. This is why swearing was used to seal a covenant or deal. When 'Jehovah swears' it does not mean that He really did swear, but that divine truth testifies or bears testimony to the things asserted.

(Odkazy: Apocalypse Revealed 474)

474. Then the angel whom I saw standing on the sea and on the land lifted up his hand to heaven and swore by Him who lives forever and ever. (10:6; 10:6) This symbolizes an attestation and testification of the Lord on His own authority.

The angel standing on the sea and on the land means the Lord (no. 470). Lifting up the hand to heaven symbolizes an attestation, here that there should be no more time (verse 6). Swearing symbolizes a testification, here that in the days of the sounding of the seventh angel the mystery of God would be concluded (verse 7). He who lives forever and ever means the Lord, as in Revelation 1:18; 4:9-10, and 5:14 above, and in Daniel 4:34. That the Lord swears on His own authority will be seen shortly.

It is apparent from this that the statement, "Then the angel whom I saw standing on the sea and on the land lifted up his hand to heaven and swore by Him who lives forever and ever," symbolizes an attestation and testification of the Lord on His own authority.

[2] That Jehovah swears or testifies on His own authority is clear from the following passages:

I have sworn by Myself; a word has gone out of My mouth (which) shall not return... (Isaiah 45:23)

I swear by Myself... that this house shall become a desolation. (Jeremiah 22:5)

Jehovah... has sworn by His soul. (Jeremiah 51:14, Amos 6:8)

...Jehovah has sworn by His holiness. (Amos 4:2)

Jehovah has sworn by His right hand and by the arm of His strength. (Isaiah 62:8)

Behold, I have sworn by My great name... (Jeremiah 44:26)

That Jehovah, which is to say, the Lord, swore by Himself or on His own authority means, symbolically, that Divine truth attests; for the Lord is Divine truth itself, and this attests of itself and on its own authority.

In addition to these passages, that Jehovah swore may be seen in Isaiah 14:24; 54:9, Psalms 89:3, 35; 95:11; 110:4; 132:11.

We are told that Jehovah swore because the church established with the children of Israel was a representational church, and the conjunction of the Lord with the church was represented by a covenant, like one made between two parties who swear to their compact. Therefore, because an oath was a part of any covenant, we are told that Jehovah swore. Still, this does not mean that He swore, but that Divine truth attests to something.

[3] That an oath was a part of any covenant is apparent from the following:

I swore an oath to you and entered into a covenant with you, so that you became Mine... (Ezekiel 16:8)

...to remember His covenant, the oath which He swore... (Luke 1:72-73; cf. Psalms 105:9, Jeremiah 11:5; 32:22, Deuteronomy 1:34; 10:11; 11:9, 21; 26:3, 15; 31:20; 34:4)

Because the covenant was representative of the conjunction of the Lord with the church, and reciprocally of the church with the Lord, and because an oath was a part of any covenant and was to be sworn on the ground of the truth in it, being sworn thus also in appeal to that truth, therefore the children of Israel were permitted to swear by Jehovah, and so in appeal to Divine truth (Exodus 20:7, Leviticus 19:12, Deuteronomy 6:13; 10:20, Isaiah 48:1; 65:16, Jeremiah 4:2, Zechariah 5:4).

After the representative constituents of the church were abrogated, however, the Lord also abrogated oaths to covenants (Matthew 5:33-37; 23:16-22).