It is impossible for God to accept any righteousness other than Christ's
It is impossible for God to accept any righteousness other than Christ's. No other human could ever meet the requirements; no person could walk the same path or pay the same price He did. Only Jesus' righteousness will stand before God: "...all our righteousness are as filthy rags..." (Isaiah 64:6).
This matter of understanding how God justifies His people is not for babes. It it is a truth for full-grown believers! Yet so many Christians are still babes, drinking milk. All they understand is forgiveness of their sins. So few go on to the meat of the Word of righteousness!
"For every one that useth milk is unskilful in the word of righteousness: for he is a babe. But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil" (Hebrews 5:13-14).
Milk is for babies -- and babies need constant attention! They feel good only when Mother is in the room. And when Mother leaves, they cry out with fear. They have to be able to see her, feel her, touch her to feel secure.
Likewise, the believer who remains on milk lives in constant dread of his salvation. He is never at ease with the heavenly Father. Instead, he's always uptight, thinking God is ready to send him to hell every time he fails. He does not know or understand his position in Christ -- and therefore he never knows the peace of God. He cannot believe that God is fully committed to save and keep him.
Paul is saying there is meat available to us -- food that will cause us to grow into maturity, to become men and women in Christ, to develop into full stature as believers. Beloved, this meat is the truth concerning Christ's perfect righteousness! Understanding it allows you to remain secure in your salvation. You don't have to fear that God will forsake you every time you slip and fall. No -- you will have victory!
But if you don't have this truth, the devil will play havoc with your feelings. He'll lie to you and push you around. Unless you have this truth as a foundation under everything you believe, nothing will be right in your doctrine, your theology, your life. You can't even obey God until you understand it!
You have to be convinced that nothing you can ever do will produce an acceptable righteousness before God. It has to come from another source! When I stand at the judgment seat, I have no other plea but the righteousness of Jesus Christ.