Albert Barnes Commentary Isaiah 65:16

"so that he who blesseth himself in the earth shall bless himself in the God of truth; and he that sweareth in the earth shall swear by the God of truth; because the former troubles are forgotten, and because they are hid from mine eyes." — Isaiah 65:16 (ASV)

That he who blesses himself in the earth - That is, he who shall invoke blessings on himself.

Shall bless himself in the God of truth - Or by the true God. He shall not seek a blessing from a false god; but he shall come before the true God, and seek a blessing from his hand.

And he who swears - Every oath that is taken in the land shall be by the true God. There shall be no swearing by idols; but the true God shall be acknowledged everywhere.

Because the former troubles are forgotten - The former punishments and calamities shall pass away. The favor of God shall be restored. His pure worship shall be re-established, and his name shall be celebrated again in the land. The image here is one of returning prosperity and favor, a state when the happiness will be so great that all the former trials will be regarded as not worthy of recollection.