It is wonderful to think how secure we are in Christ, all because of His finished work. His finished work not only took away all my sin, so that I am never seen as a sinner anymore, and therefore never judged, condemned, or punished - but His finished work also made me the righteousness of God in Christ! Now THAT'S righteous! That's a righteousness that can never fade or be marred or stained. That's perfect righteousness! That's unchanging righteousness! That's eternal righteousness!
This perfect righteousness is based on a perfect sacrifice as part of a perfect covenant made and upheld by God and Jesus (and not up to me to uphold or keep it)!
So it's a secure and unchangeable righteousness because it isn't based on me and my working and ability to keep things going but based on Christ and His doings and His ability to keep things going. He ever lives to intercede for me and that means the righteousness He has given me will last forever!
Christ's sacrifice was once-for-all and lasts for all time. By one offering He has perfected forever those who are sanctified. By His sacrifice we are perfected once for all time! And since His sacrifice perfected me once for all, for all time, I am righteous before God for all time!
This righteousness isn't just a mask that I wear to cover up my ugly self that still exists underneath - no, I have been made the righteousness of God! I have been reborn as a righteous person! I have been infused with His righteousness at the very core of my being through being regenerated/recreated as a new creations with the Holy Spirit as my very life! My spirit has been recreated in union with His Spirit - they are one. All that He is has been infused into my spiritual DNA. I have the very holiness of the Holy Spirit at my core, in my new nature. I have been made a partaker of the divine nature! Now that's holy! Set apart for Him, beautiful, clean and pure, good and lovely and new! Innocent and also perfect before Him - a saint (holy one, consecrated one). I'm never a sinner again but always a saint in Christ Jesus! That's what has happened and is now forever true of my reborn spirit! That's a wonderful righteousness!
And another beautiful way to look at it is this: righteousness just means a state of rightness, being in right relationship with God. So for us to have been made the righteousness of God in Christ means that we have been put in perfect and right relationship with God, as perfect a relationship as God the Father and His Son have! We have been brought into the midst of that very relationship and have been made to partake of it, enjoying the same fellowship and love. Jesus said that as the Father has loved Him, He has loved us. He prayed that the very love of the Father that was in Him would be in us, and that is what He has accomplished through His death to take away sins and through the resurrection and new birth, putting His very Spirit in us. We have been brought into the family love circle of the Trinity!
We are safe and secure in His eternal hug! The Father won't ever stop loving Jesus, and because we've been brought into that same right relationship that Jesus has with the Father, the Father won't ever stop loving us! We now have this perfect love relationship with Him, which is what He wanted all along for us! And we have it irrevocably and permanently because we have been made the unchangeable, perfect righteousness of Christ!