There’s a reason why historic Reformed/Presbyterian churches respond so quickly, strongly, and ecclesiastically (i.e. in a churchly manner) to preaching that clouds the gospel by mixing law/works and gospel/grace. When we hear things like “works are instrumental in justification” or “final justification at the last day” or “the lawful gospel” or “I’m not sure ‘imputation’ is the best way to talk” or “faith alone means being faithful to Christ’s call” and other confusing statements, we investigate because these types of statements bring on a theological fog that so quickly confuses God’s people about the heart of the faith, the gospel. Concerning this, Martin Luther made the following outstanding observations in his commentary on Galatians 1:6 (I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel… NASB). By the way, these statements are totally applicable today in light of the Federal Vision and New Perspective(s) on Paul:
Paul complains that it is easy to fall (compare 1 Corinthians 10:12). We, too, find in everyday experience how hard it is for the mind to conceive and retain a sound and steadfast faith. It may take ten years’ labor before a little church is properly ordered; then some lunatic gets in who can do nothing but speak slanderously and spitefully against sincere preachers of the Word, and in one moment he upsets everything. That happened with Paul, the chosen instrument of Christ. He had won the churches of Galatia with great care and labor, and the false apostles, shortly after his departure, overthrew it, as this and other letters prove. So great is the weakness and wretchedness of this present life. We walk among Satan’s snares, and one person with mad ideas may destroy in a short time all that has been built up over many years by many true ministers laboring night and day. We learn this from experience, with great grief; yet we cannot do anything about it.
Since the church is such a soft and tender thing, and so soon overthrown, we must be quick to watch against these people with their mad ideas. When they have given two sermons or have read a few pages of the Holy Scriptures, they reckon they are in control of all learners and teachers and are answerable to no human authority. You can find many such people today, bold and impudent persons who because they have not been tried by temptations have never learned to fear God, nor had any taste or feeling of grace. Because they are empty of the Holy Spirit, they teach what they like best and such things as are plausible and pleasant to the common people. Then the uneducated multitude, longing to hear news, soon joins them. And many others who think themselves well versed in the doctrine of the faith and have been tempted to some extent are seduced by them.
Paul teaches us from his own experience that congregations that are won by great labor are easily and soon upset. We should watch very carefully against the devil’s rangings everywhere, lest he come while we are asleep and sow weeds among the wheat. However watchful and diligent the shepherds may be, the Christian flock is in danger from Satan. Let us therefore watch carefully—first, every one for himself, and second, all teachers, not only for themselves, but also for the whole church, so that we do not enter into temptation.
The above quote is found in Martin Luther, Galatians, Crossway Classic Commentaries (Wheaton, IL: Crossway Books, 1998).