From the Reformer’s Commentary on (the third verse of the first chapter of) Galatians:

Verse 3. Grace be to you, and peace, from God the Father, and from our Lord Jesus Christ.

The terms of grace and peace are common terms with Paul and are now pretty well understood. But since we are explaining this epistle, you will not mind if we repeat what we have so often explained elsewhere. The article of justification must be sounded in our ears incessantly because the frailty of our flesh will not permit us to take hold of it perfectly and to believe it with all our heart.

The greeting of the Apostle is refreshing. Grace remits sin, and peace quiets the conscience. Sin and conscience torment us, but Christ has overcome these fiends now and forever. Only Christians possess this victorious knowledge given from above. These two terms, grace and peace, constitute Christianity. Grace involves the remission of sins, peace, and a happy conscience. Sin is not canceled by lawful living, for no person is able to live up to the Law. The Law reveals guilt, fills the conscience with terror, and drives men to despair. Much less is sin taken away by man-invented endeavors. The fact is, the more a person seeks credit for himself by his own efforts, the deeper he goes into debt. Nothing can take away sin except the grace of God. In actual living, however, it is not so easy to persuade oneself that by grace alone, in opposition to every other means, we obtain the forgiveness of our sins and peace with God.

The world brands this a pernicious doctrine. The world advances free will, the rational and natural approach of good works, as the means of obtaining the forgiveness of sin. But it is impossible to gain peace of conscience by the methods and means of the world. Experience proves this. Various holy orders have been launched for the purpose of securing peace of conscience through religious exercises, but they proved failures because such devices only increase doubt and despair. We find no rest for our weary bones unless we cling to the word of grace.