There are two ‘forevers’ in these two verses.
God’s word is settled forever
God’s faithfulness endures forever
This reminds me of Paul’s admonition to the Corinthians that they ‘don’t look on the things you can see, but look at the things you cannot see. The things you can see are on temporary, but the things you cannot see are for all eternity.’
Everything we can see will be gone one day – but God’s word lasts for ever and He is faithful for all eternity.
We have plenty of examples of that. We read that Jesus was faithful to death – even the death on the cross. We read that Jesus saves to the very end those who put their faith in Him. We read that Jesus is the ‘author and the finisher of our faith.’
There is a place where we find a consistency that lasts to the very end. God is as real and faithful to His word as He was when He said ‘let there be light.’ His word will endure no matter what happens along the way.
God made the earth and it still endures to today. That is the evidence we need to know that His word endures.