“For this reason the distinction between the law and Gospel is retained. The law does not promise freely, but under the condition that you keep it completely. And if someone should transgress it once, the law or legal covenant does not have the promise of the remission of sins. On the other hand, the Gospel promises freely the remission of sins and life, not if we keep the law, but for the sake of the Son of God, through faith.”
“The sum of the Gospel Paul taught is this: we are justified or receive the remission of sins by faith alone in Christ. It is the Pseudoapostles who confuse circumcision and the merit of works…. It is worth observing how often merit, whether from the sacraments or other works, is confused with faith. That is another gospel, not of Christ, but rather a perversion of the ground of the gospel itself.”
Great quotes there; they can be found (in English) in chapter six of Scott Clark’s Caspar Olevian and the Substance of the Covenant. If you want an excellent commentary of Olevian’s on the first 24 Lord’s Days of the Heidelberg Catechism, you’ll need to get A Firm Foundation (though hunt around for a cheaper copy than Amazon’s). In that commentary Olevian explains the law/gospel distinction in further detail and also shows what it has to do with true comfort in the Christian life.