In order to offer and convey to men the merits which Christ has secured for the world by His death on the cross, 2 Cor. 5, 21; Rom. 5, 18, God employs certain external, visible means through which the Holy Spirit works and preserves faith and thus accomplishes the sinner's salvation.
That is the clear teaching of our Confessions. The Formula of Concord thus writes XI, 76: "The Father will not do this draw any one to Himself without means, but has ordained for this purpose His Word and Sacraments as ordinary means and instruments." The Smalcald Articles Part III, Art. VIII, 3: "In those things which concern the spoken, outward Word we must firmly hold that God grants His Spirit or grace to no one except through or with the preceding outward Word." The Augsburg Confession Art. V, 2: "They our churches condemn the Anabaptists and others who think that the Holy Ghost comes to men without the external Word, through their own preparations and works."
Our dogmaticians define the means of grace as "media externa a Deo ordinata, quibus Deus gratiam a Christo acquisitam hominibus offert et fidem ad gratiam accipiendam necessariam in hominibus efficit et conservat." As divinely ordained means of grace they acknowledge, on the basis of Scripture, only the Word (the Gospel)1 and the Sacraments, Baptism and the Lord's Supper, the latter two as the visible Word (Verbum visibile).
According to Scripture these divinely ordained means have a twofold function or power, namely, a) an exhibiting, offering, or conferring power (vis exhibitiva, dativa, collativa) and b) an effective, or operative, power (vis effectiva sive operativa). The first consists in this, that the Holy Spirit through the means of grace earnestly offers to those who hear or read the Word the grace of God (Dei favor) and the righteousness of Christ (meritum Christi); the second, that He through the means of grace actually works, strengthens, and preserves in the hearts of men a living faith in the gracious forgiveness of their sins, so that they are converted, justified, sanctified, and finally glorified. For this reason we rightly call the means of grace media communicationis remissionis peccatorum sive iustificationis ex parte Dei.
This doctrine of the means of grace the conceited reason of man has corrupted in a twofold manner. On the one hand, it has declared that means of grace are unnecessary for salvation (Zwinglianism: The Holy Spirit requires no wagon vehiculum for His divine operations); and, on the other, it has added to the two Sacraments ordained by Christ additional sacraments (Romanism: penance, confirmation, marriage, the ordination of priests, extreme unction). In the final analysis every perversion of the doctrine of the means of grace is made in the interest of the doctrine of work-righteousness.
Pieper discusses in the chapter on the means of grace if prayer is a means of grace, page 253-258 etc.
Theologians after Luther's death have analyzed what Luther has taught (and written on 22,446 pages), and summarized it in Christian doctrines.
Let's read (and hear) how Luther himself summarized the teachings of the Bible and his own teachings in a quote that is widely used by Lutherans around the world:
"For three things are necessary for a man to know that he may be saved:
First, that he should know what he ought to do and what he ought not to do;secondly, if he should see that he cannot do or leave it by his own strength, that he should know where to take, seek, and find, that he may do and leave the same;thirdly, that he should knowhow to seek and have it." (Vol. 9 of Luther's works, page 9.)