I would imagine you read my previous post where I stated I believe the Christian should be in tension. The simple truth is the Pharisees believed they were sincere in observing the law, and yet they were not. The difference then is significant - simply stating that one is sincere in their piety does not sincere piety make, as scripture attests. It is the heart of the worshipper ... the spirit, in "spirit and truth." One would be assuming very much to imagine that I am opposed to a serious study of the law, or a sincere desire to keep it. Likewise one would be assuming much to imagine that serious study of the law and a sincere desire to keep it is a sure defense against becoming a white washed tomb. That same heart that is a cesspool of unrighteousness will just as surely twist the law, as it will license and liberty - is that not the testimony of Christ in all of His dealings with the Pharisees? They have the law, but their hearts were far from Him. I will simply repeat: I believe the Christian should live in the tension, and not become complacent in imagining that their sincere devotion to the commandments of God is somehow different that that of the Pharisees. It is the heart of the one following the law that reveals the motives of their law keeping - keeping the law itself is no guarantee that the motive within is pure.
One's definition of legalism must acknowledge subjectivity due to the corruptness of the human heart. One can feel very strongly that the Lord has commanded, or due to necessary and reasonable consequence, a practice of worship. They can validate that with their confession and tradition, and even the testimony of good Godly counsel. And yet legalism is when the human heart elevates the law over worship, when the object of worship in the heart of the person is law-keeping, and not the law-Giver. A tell-tale sign of this subtle form of idolatry, is when it matters most to them when others are not law-keeping. The serpent is the most subtle of all creatures, and our hearts are prone to subtle sins, particularly when we consider ourselves most knowledgeable of righteousness. Pastorally then I implore you to examine your hearts constantly in your pursuit of "right worship" for the subtle shifts that make idols of the very laws God gave us for our good and His glory. I'd rather you always question yourself, be uncomfortable, lean on mercy and grace, than to risk feeling settled and wind up with the Pharisees. Their worship was distinctly absent of any God orientation, or need for God in worship, even though it observed all the forms commanded, and was highly regulated.