He said to them, “But who do you say that I am?” Simon Peter answered and said, “You are the Christ, the Son of the living God.” Jesus answered and said to him, “Blessed are you, Simon Bar-Jonah, for flesh and blood has not revealed [this] to you, but My Father who is in heaven. And I also say to you that you are Peter, and on this rock I will build My church, and the gates of Hades shall not prevail against it. Matthew 16:15-18 (NKJV)
The revelation of Jesus as the Christ is the foundation of life.
All that the eternal unchanging God is and has permanently dwells in Christ. Colossians 1:19 (AMPC) For it has pleased [the Father] that all the divine fullness (the sum total of the divine perfection, powers, and attributes) should dwell in Him permanently.
Therefore when you talk about unchangeability and permanency then you’re referring to Christ. He’s the true solid rock on which we stand.
Everything unseen and seen was created through and is sustained by Christ. See- Colossians 1:16, 17. It is only in Him that we can never be moved or shaken.
Christ as solid rock means we can confidently stand and rely on who He is and what He has done for us because everything about Him is everlasting. We can also be assured that our identity and position in Him is unshakable and can never change.
Our forgiveness in Him, our salvation in Him, our being made right with God in Him, our being made holy in Him, our acceptance before God in Him are all unchangeable and permanently established forever.
This is what gives us rock solid assurance and confidence.
Christ the author and finisher of our life, faith, salvation is rock solid.
Therefore we have, live and walk a rock solid life, faith and salvation.