One way we easily slip into legalism is through confusing biblical principles and the methods we use to uphold those principles. Recently I posted a story called The Legalistic Steward which illustrated this problem in action. The steward in that story, in an effort to accomplish the instruction (the principle), of his employer devised means (methods), which soon became ends (principles) in and of themselves. In the process the original instruction and its purpose were completely lost.
At the risk of stepping on a few toes (Be gentle, gentle readers!), let me give a few examples where I think we do this.
We are given principles in Scripture about the education of our children. It is to be God-centered. It is the responsibility of the parents. It has as one of its goals to give our children a thorough knowledge, understanding, and wisdom about the Scriptures. There are many methods parents use to accomplish these goals. There are different school situations (home school, tutor, Christian day school). There are different approaches (Classical Method, Principle Approach). Do we so exalt one method above another that we make it the principle? I think it's great to discuss different methods and support one or another because you think it best meets the biblical mandates, or at least is best for your situation. I think it is also useful to point out why a certain method does not meet the biblical goals or is based on a flawed philosophical foundation, (i.e. Montessori, which is based on the humanistic foundation that children are inherently good, desire to learn, etc.). But sometimes I hear such things as, "Homeschooling is THE only biblical way to educate children," or "The Classical Method is THE biblical approach." In this we err by turning good methods into principles.
I wrote a post on modesty recently. While I did end it with some suggested methods (clearly labeled as such), I think this is another area in which we need to look more closely at the principles than we often do. A quick internet search on this topic will yield many articles that only lightly address the principles, (be they purity, respect for marriage, or expressing femininity) and are heavy on the methods. Sometimes these lists of methods include things that it may be argued are good ways to meet the principles, but can hardly be held up as Biblical proscriptions. Issues such as skirt length, women wearing pants, hair length, etc., are often presented with either dubious scriptural support or none at all. Again, it's fine to argue for these methods to uphold biblical principles, but we go too far if we say things must be done in this way.
Just in case I haven't offended everyone yet, let's consider the use of alcohol. The Scriptures are clear that drunkenness is forbidden. Drunkenness is described as situations in which people lose self-control, fall into sin, and/or speak foolishly. (For example, Noah, in Genesis 9) However, this is not the same thing as the 'glow' that is the natural effect of drinking a moderate amount of alcohol. This is spoken of positively. (For example in Psalm 104:15) This was the situation at the wedding in Cana, (when the guests had, "well drunk"), that Jesus found it appropriate to make more wine! (John 2) Avoiding all use of alcohol is certainly an effective method of avoiding drunkenness, and for some people it may the only way that works for them. But we often take this too far by making the avoidance of all alcohol the principle in place of the scriptural principle to avoid drunkenness.
We are told that the law of God is perfect. (Psalm 19:7) When we change something that is perfect, do we make it better or do we make it worse? Yet how often do we see what the law of God says and think to ourselves that we should not just obey the law, but we should do even better?
I think the greatest danger in pietism or legalism is the effect it has on new believers. Are mature Christians modeling a reliance on the work of Christ when we "improve" on His law, or are we encouraging a reliance on our own works? When we uphold standards that cannot be supported biblically, and insist that others uphold them (or simply imply that they should), what are we saying about the Bible as our standard for faith and practice? Are we leading new believers into the practice of hypocrisy because they have to pretend they agree with these rules or pretend they keep these rules in order to be good enough to be in our company? Are we causing a weak brother to question his salvation because he drinks a beer or sends his kids to Christian day school?
Another issue with new believers is humility. Probably most of us who have been Christians for a long time can testify that even though the sanctifying work of the Holy Spirit has made us more Christ-like than we once were, we are also more painfully aware of how unChrist-like we still are. The more sanctified we get, the less we esteem ourselves and the more we esteem Christ. I believe that pietism can tempt the new believer to look about, believe himself better than others who do not meet these standards, and therefore be hampered in developing that humility that becomes a Christian.
Romans 14 is often used as a reason why higher standards should be upheld, especially in such things as abstaining from alcohol. The ironic thing is that this chapter, taken as a whole, is a strong statement against making our methods into pietistic additions to God's law and holding others to those standards. "Let each be fully convinced in his own mind. He who observes the day, observes it to the Lord; and he who does not observe the day, to the Lord he does not observe it."
These instructions are moderated with statements that tell us that if our brother is grieved or may stumble because of what we are free to do, that it is loving to refrain for his sake. However, I would question the assertion that this means we ought to refrain at all times because of some chance that someone may at some time be grieved by it. Such a practice, I believe, may actually make our brothers stumble for the reasons I noted above.