2 Cor. 3:1-6 (ESV)
Are we beginning to commend ourselves again? Or do we need, as some do, letters of recommendation to you, or from you?  You yourselves are our letter of recommendation, written on our hearts, to be known and read by all.  And you show that you are a letter from Christ delivered by us, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.
 Such is the confidence that we have through Christ toward God.  Not that we are sufficient in ourselves to claim anything as coming from us, but our sufficiency is from God,  who has made us competent to be ministers of a new covenant, not of the letter but of the Spirit. For the letter kills, but the Spirit gives life.
The confidence of the Pastor to preach both the law, and the gospel must be found in Christ. I am neither perfect enough in myself to condemn sinners or forgive them. A true pastor finds his sufficiency in God, from God. He has made us poor miserable sinners, to be ministers of his New Testament, the forgiveness of sins.
It is a New Testament. The Old Testament was the promise sealed to Abraham in circumcision. I used to think of it as a covenant, but no longer. The testament was a promise made to Abraham, yet to be fulfilled. But it was a promise from God. The promise was made before the circumcision, it was made before the giving of the law. It was a one sided promise. The giving of the law came 430 years later. The New Testament is the fulfillment of that The Old Testament in the death and resurrection of Christ.
The problem is if we are to say it was an Old Covenant, and a New Testament. Then the Israelites, and Abraham, were saved by the law, and not by faith. Abraham though was saved by faith, not by the law. The Israelites were also saved by faith in the promise given to Abraham, not by the law. We are not dispensationalists who believe God changes His plan of salvation according to times or people. Rather all people of all times are saved in the same way, through faith in the promise given to Abraham, which is fulfilled in Abraham. Therefore, all us stones, are made children of Abraham through faith, and participate in the inheritance of Abraham, which is the eternal life.