Wonderful question! The answer begins with an understanding of the meaning of "covenant." Many see Jesus on the Cross and think in juridical terms (condemned by our sins) or even contractual terms (He paid the price), and these are true... but insufficient. He is the sacrificial offering of the New Covenant. Covenant differs from contract as much as marriage differs from prostitution. One is family-making, the other is a business arrangement.
Since our fall in the Garden of Eden, God has been rebuilding His Family... following the same model as our Creation. In the Creation account, He worked by days providing first the form and second the inhabitants. In the first three days, He separated light from dark (day one), separated the waters above and below (day two) and gave us dry land with vegetation (day three). He then filled these realms; sun and moon (day four), birds and fish (day five), and animals and man (day six). He rested on the seventh day not out of fatigue but to mark his creation with a covenant... the number seven is a covenantal number. We broke this covenant through disobedience and so He set out again...
God made His first starting-over covenant with Noah and his family with the seven colors of the rainbow. God then made His family-rebuilding covenant with Abraham in the form of a three-fold promise... the land, a nation and all men blessed through him. God affirmed His covenant with Isaac (because Abraham had another son, Ishmael). God confirmed His covenant again with Jacob (because Isaac thought to pass his blessing through Esau). In these promises, the unbroken bloodline matters in fulfilling God's promise, but it is God, not the Earthly succession Who confirmed the line His promise would follow. God then began to fulfill these promises... the land through Moses on Sinai, the royal dynasty of David, and all people through Christ on the Cross. Seven covenants just like the seven days of creation.
In each successive covenant, God is growing his family... from the family of Noah to the tribe of Jacob to the Nation under David and then all nations under Christ. What was Christ doing on the Cross? He was the sacrificial offering of the New Covenant where man could be rejoined to God's family. Just as the neighbor kids can't become part of my family on their own--I have to adopt them--so we couldn't make ourselves part of God's family. The New Covenant is a return to our participation in the family life of God!
Just as we don't delight in the accomplishments of the neighbor kids, so God did not delight in our vain offerings before our redemption. Now, through Christ, God does delight and reward our efforts. In so many places in the New Testament we find that God will reward us for our deeds. That is because Christ has merited for us the ability to merit from God.
Christ on the Cross was at once Priest offering a sacrifice, the sacrificial Lamb of the New Covenant, and the Bridegroom to His Church. Now, in order to be part of His Family, we are commanded to 'believe.' However, believe is an action word. It doesn't mean that we have an intellectual understanding... just as my kids understand that school is important for them--but it doesn't do them any good until they actually go to school and participate. If we BELIEVE that Jesus is Christ, then we are to DO the Will of His Father... which begins our life in God's Family through baptism in His Name.
Christ on the Cross isn't the end from which there is no more to be done, it is the beginning from which all else is possible.
I think we are mixing definitions here. I don’t think there are any Protestants anywhere who will deny that belief results in positive action - i.e. proof of salvation. That is indeed the cornerstone of our argument. Belief is not some passive thought process. A saving faith is the beginning of a relationship with our Lord, but our salvation is not dependent on somehow not screwing up. We can’t earn it, didn’t earn, won’t ever earn it. It’s grace - a gift. We can’t lose what we haven’t earned. That’s why only God can do the job.
Applaud and many blessings for your great post.