The importance of the difference between Law and Gospel

It is very important to know the difference between the two principle points of Christianity or else one would become liable to the charge of legalism or licentiousness.

"By 'the law' here is meant the moral law as a covenant of works, and by 'the gospel' is meant the gospel in its strict and proper sense. To know the differemce so as to be able to distinguish aright between the law and the gospel is of every itmost importance to the faith, holiness, and comfort of every true Christian. It will be impossible otherwise for a man so to believe as to 'be filled with joy and peace in believing.' If he does not know the difference between the law ...and the gospel he will be apt, especially in affair of justification, to confound the one with the other. The consequence will be that his painful experience, bondage will be mixed with liberty of spirit, fear with hope, sorrow with joy, and death with life. If he cannot so distinguish the gospel from the law as to expect all his salvation from the grace of the gospel, and nothing of it from the works of the law; he will easily be induced to connect his own works with the righteousness of Jesus Christ in the affair of his justification. This was the great error of the Judaizing teachers in the churches of Galatia. They mingled the law with the gospel in the business of justification, and thereby they so corrupted the gospel as to alter the very natof it and make it another gospel. They taught that unless men were circumcised and kept the law of Moses, they could not be justified or saved (Acts 15:1-5). They informed the people that while the righteousness of Christ received by faith was necessary, their own works of obedience were also requisite in connection with it to entitle them to justification before God. This is a fundamental error, and such a one that if even an angel from heaven would publish it he should be accursed." -John Colquhoun, A Treatise on the Law and Gospel

"The principal DIFFERENCES between these two parts of the doctrine of the church, consist in these three things:

1. In the subject, or general character of the doctrine, peculiar to each. The law prescribes and enjoins what is to be done, and forbids what ought to be avoided: whilst the gospel announces the free remission of sin, through and for the sake of Christ.
2. In the manner of the revelation peculiar to each. The law is known from nature; the gospel is divinely revealed.
3. In the promises which they make to man. The law promises life upon the condition of perfect obedience; the gospel, on the condition of faith in Christ and commencement of new obedience. Hereafter, however, more will be said upon this subject in the proper place." - The Commentary on the Heidelberg Catechism by Zacharias Ursinus