It has been my experience that there are two kinds of legalism. One kind earned the Pharisees Jesus's most scathing rebukes. The other has pushed many in the modern church to the precipice of heresy... and beyond.
The legalism that made the Pharisees infamous is a kind of pre-salvation legalism and is built on the notion that one can earn his way out of his condemned position before God. It is "pre" because salvation is received only after a completed obedient life of works. However, according to scripture, the law was never meant to make one righteous, nor was it meant to lay out a road map by which one could earn his own salvation. On the contrary, it was meant to show that man is a sinner and is hopelessly unable to live up to the letter of the law. [1](Rom 7:7-13)
The legalism of our day on the other hand is a kind of post-salvation legalism that results in what is known as antinomianism. Antinominanism is based on the idea that once one has entered into salvation there is no moral law, only grace. It qualifies as legalism only in that it outlaws law. Instead of salvation being the result of obedience to myriad laws, it is now predicated on obedience to one: do not put anyone under the law. Not surprisingly, as with all bad doctrines and heresies, both legalisms have similar effects on the people to which they are taught. They both can be proof texted. They both lead to pain and suffering. They both are a result of bad doctrine. They both cause those who preach them to become hypocritical. They are both man centered. They both give their ardent adherents a sense of pride that causes self-righteous anger when challenged.
But possibly the most important similarity between these two legalisms is that they are both issues of the heart. Following Paul's discussion on the matter of legalism vs. grace in Romans 7, we get this wonderful picture of the war that wages in the heart of the true seeker of God. [2](Rom 7:13-25) This act of war raises what I think is a very pertinent question: If we are not obliged to the law, then why bother joining with Paul in his warring against his own members? The answer to this question can only be found by an examination of our hearts.
One of the evidences that the believer enjoys that reassures him that he is in fact saved is the brokenness he feels in his heart when he inevitably breaks the law. But it is in times like this that we discover that the salve of grace is not cheap, as some might suppose, but rather is a genuine treasure. It is through this grace that we experience our greatest joy in the Lord and our greatest gratitude that on account of our lack of ability to follow God's law our eternal fate is not sealed. [3](Phil 4:4-7) The realization of this truth is what transforms our salvation from a free, as in cheap, insurance policy to a priceless gift. But all of this is cheapened when there is no law by which to abide which means there is no threat being leveled against our mind from the members of our own body. [4](Rom 7:23) But there is a cumulative effect that follows this scriptural error as consequence follows consequence. One such effect is the onset of confusion. Jesus lived a perfect life; a life without sin. This necessarily demands that there was a perfect standard by which he was required to live. The confusion comes when we confuse our attempts to live the perfect life Jesus commanded us to live with our salvation. One type of legalism puts our salvation at stake with each failure, the other outlaws the very standard by which Jesus was declared righteous. Further, as consequence continues to follow consequence, it shouldn't surprise anyone that this predominate scriptural error has created, among many things, an environment by which relativism thrives, both in and out of Christendom.
[1] Rom 7:7-13 7 What shall we say, then? Is the law sin? Certainly not! Indeed I would not have known what sin was except through the law. For I would not have known what coveting really was if the law had not said, "Do not covet." 8 But sin, seizing the opportunity afforded by the commandment, produced in me every kind of covetous desire. For apart from law, sin is dead. 9 Once I was alive apart from law; but when the commandment came, sin sprang to life and I died. 10 I found that the very commandment that was intended to bring life actually brought death. 11 For sin, seizing the opportunity afforded by the commandment, deceived me, and through the commandment put me to death. 12 So then, the law is holy, and the commandment is holy, righteous and good. 13 Did that which is good, then, become death to me? By no means! But in order that sin might be recognized as sin, it produced death in me through what was good, so that through the commandment sin might become utterly sinful.
[2] Rom 7:13-25 13 Did that which is good, then, become death to me? By no means! But in order that sin might be recognized as sin, it produced death in me through what was good, so that through the commandment sin might become utterly sinful. 14 We know that the law is spiritual; but I am unspiritual, sold as a slave to sin. 15 I do not understand what I do. For what I want to do I do not do, but what I hate I do. 16 And if I do what I do not want to do, I agree that the law is good. 17 As it is, it is no longer I myself who do it, but it is sin living in me. 18 I know that nothing good lives in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. 19 For what I do is not the good I want to do; no, the evil I do not want to do-this I keep on doing. 20 Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it. 21 So I find this law at work: When I want to do good, evil is right there with me. 22 For in my inner being I delight in God's law; 23 but I see another law at work in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members. 24 What a wretched man I am! Who will rescue me from this body of death? 25 Thanks be to God-through Jesus Christ our Lord!
[3] Phil 4:4-7 Rejoice in the Lord always. I will say it again: Rejoice! 5 Let your gentleness be evident to all. The Lord is near. 6 Do not be anxious about anything, but in everything, by prayer and petition, with thanksgiving, present your requests to God. 7 And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus.
[4] Rom 7:23-24 23 but I see another law at work in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin at work within my members.