What is the proper doctrine of the Gospel, as distinguished from the Law?

 ... the answer is easy and simple as to what is the proper doctrine of the Gospel, which is to be distinguished from the Law, seeing that it speaks of the benefits of Christ and reveals the righteousness of faith before God.

Therefore we shall divide these passages into certain categories in order to expedite their explanation.
1. The Gospel is preached to those who are repentant, and it deals with the gracious promise of reconciliation, remission of sins, righteousness before God, salvation, and acceptance unto eternal life. This promise is established in God’s grace, mercy, and love, Eph. 1:6–9; 2:8; 1 Tim. 1:15; Titus 2:11.
2. In defining the Gospel we must always include the person of Christ, in His office as Mediator. For “in Him all the promises of God are Amen,” 2 Cor. 1:20. “The covenant is confirmed in Christ,” Gal. 3:17.
Here it is absolutely necessary that the benefits of Christ on account of which we receive remission of sin and are received unto eternal life be distinguished from the benefits of sanctification or renewal which follow justification. We are not justified for the sake of the latter, that is, we do not receive remission of sins nor are we received unto eternal life because of this renewal which follows justification, although it is also a benefit of Christ.
The benefits of reconciliation are these:
(1) Christ has taken upon Himself our sins and the punishments for our sins and has made satisfaction to the Father for them.
(2) He “is the fulfillment of the Law for righteousness to everyone who believes,” Rom. 10:4. Because of this benefit of Christ, we believers are reconciled to God and received unto eternal life.
(3) The Gospel teaches that these benefits of the Mediator are apprehended by faith and apply to those who repent.
(4) The Gospel teaches that these benefits are offered through the Word and the sacraments, through which instruments the Holy Spirit is efficacious, illumines our hearts, works faith, pours life-giving consolation into our hearts, raises them up, and sustains them.
(5) After the benefit of grace or justification the Gospel also contains the promise of “the free gift through grace,” Rom. 5:15, or of the “truth,” John 1:17, namely that the Spirit of renewal is poured into believers, the Spirit who writes the Law in their hearts so that we may be “His workmanship, created in Christ Jesus for good works, which God prepared in advance, that we should walk in them,” Eph. 2:10.
(6) The Gospel speaks not only of present benefits we receive by faith for the sake of Christ in this life, but it has also the promise of “the hope of the righteousness which we await,” Gal. 5:5, where “God will be all in all,” 1 Cor. 15:28, and the hope of “the glory of God which shall be revealed in us” in the life to come, Rom. 5:21; 8:18–21. (7) The promise of the Gospel is universal, pertaining to all, both gentiles as well as Jews, who repent and receive the promise by faith.
These fundamental points regarding the distinctive doctrine of the Gospel are true, and it is necessary that they be diligently retained in the church; for otherwise the purity of the doctrine of justification cannot be preserved. If some insane notion is admitted regarding this question, then there will immediately follow a corruption of the article of justification ...

Chemnitz, Martin ; Preus, Jacob A. O.: Loci Theologici. electronic ed. St. Louis : Concordia Publishing House, 1999, c1989, S. 450