The practical implication of this becomes clear when we remember that Psalm 102 is possibly the most eloquent description of depression and despair to be found in the entire Psalter. The psalmist's mental salvation lay in his rediscovery of the immutability of God. Hebrews gives that truth flesh-and-blood dimensions in Jesus Christ. You can trust Him; He is always the same.
Do not mistake the meaning. This is not the immutability of the sphinx -- a Christ captured once for all in a never-fading still photograph. This is the changelessness of Jesus Christ in all His life, love, holiness, grace, justice, truth, and power. He is always the same for you, no matter how your circumstances change.
Say this to yourself when you rise each day, when you struggle, or when you lay your head down sadly on your pillow at night: "Lord Jesus, You are still the same, and always will be."
The immutability of Christ is the changelessness of the Christ revealed in the Gospels. All that He proved to be in His ministry is an indication of the way He really and always is. That is why it is legitimate for us to see the Gospel accounts not only in the context of redemptive history but as portrayals of the character of the Christ who lives forever. We are able to say, "If Jesus was like this then, Jesus is like this now."
Do you know the Christ of the Gospels? Or have you fallen into the trap to which Christians (especially, perhaps, Reformed Christians) who love doctrine and systematic theology are sometimes susceptible (unlike John Calvin, it should be said): fascination with dogmatic formula at the expense of love for the savior's person?
-Sinclair Ferguson, In Christ Alone, p. 66-7