Revelation 10:5Meaning & Commentary from the Church Fathers

Since God can swear by no one higher, He swears by Himself (see Heb 6:13). Angels, being creatures, confirm what they have said with an oath by the Creator, because of our unbelief. The angel swears either that in the age to come there will be no more time, since there truly will be no time measured by the sun, but eternal life surpassing temporal reckoning. Or he swears that after the six voices of the angel, only a little time will pass before all the foretold things are fulfilled.

The angel swears by the name of God, by Whom he was sent, that there will be no more time for repentance or for bringing forth its good fruits.

Chapter 10, verses 5-6 confirm the legitimacy of using an oath.