I take it for granted that you understand that Judaism is IN THE MAIN largely concerned with the explication and observance of law with additional exegesis of that law in the form of the Talmud. What I'm about to say is in no way intended to be pejorative: Judaism is, in some fundamental way, legalistic.
"Fundamental" is a very strong word and I don't believe from what I have learnt about Judaism that the "legalism" is really so strong to the point that it becomes "fundamental." I think it may be a misunderstanding of the way the Jewish thought system works.
I think the idea that Judaism is "legalistic" just because it revolves around a "Law System" is a very simplistic view. Actually, I would like to argue that a religion without a Law System can become legalistic just as easily. Look, for example, what has happened to Christianity. Christianity may not have a Law System and for that reason it cannot be legalistic with regard to a Law System, but that doesn't mean it can't be legalistic.
Consider, for example, how fragmented Christianity has become over the last 2,000 years with different groups arguing and splitting off over theological disputes and differences. People who disagree with the mainstream, orthodox or established position are sidelined, condemned, demonised and expelled as heretics and worse -- labelled as followers of the devil.
What drives these internal conflicts has always been the question of "what is truth?" The reason why people cannot agree is because they have different interpretations. Their interpretations are mutually contradictory and the contradiction creates a feeling of a crisis. When there is a crisis there is a fear that the "truth" has been lost and this provokes people to engage in a struggle to restore the "truth."
The Christian Gospel was supposed to be powerful and this belief in the power of the Gospel provoked people into a frenzy of fear that somehow the truth was lost in the midst of a crisis of theological disagreement.
Jesus said "love your neighbour as yourself." If that was the purpose of the Law and God was satisfied with that, then the last 2,000 years of theological bickering was futile and pointless. If the Pharisees nitpicked on unimportant concepts, then Christians aren't much better because they have repeated that same sin.
Legalism isn't about whether you have a Law System or not. It's how you approach your thought system that can make it legalistic. Jews call their 613 commandments a Law System, but they have found a way to avoid "legalism." Christians don't call their theology a "Law System," but it has certainly been the cause of much bickering. Theology has become a "law unto itself." Ultimately, "law" is just a word. "Law system" and "theology" are just thought systems by which you separate "right" from "wrong."
It is important to understand that what qualifies as "legalism" can be quite subjective, just like what qualifies as "idolatry" is subjective.
As an outside observer looking into a religious community whose tradition you don't understand, you may identify certain behaviours as "legalistic" or "idolatrous" simply because they give so much devotion and attention to it. To really be sure, however, you have to go into that community, ask the people and find out how they think. You have to find out how they see their own tradition and how their people relate to each other on religious matters. Are they humble and gentle towards each other?
Corporal Jake Sully infiltrates the Na'vi in James Cameron's Avatar and learns their culture inside out. What he initially thinks is a primitive tribal community is actually much more complex.
What I regard as legalism is when you are arrogant, self-righteous, dogmatic and judgmental
on the basis that they
do not conform to your chosen thought system. People can still be nice and understanding despite the failure of others to conform. There is nothing to suggest that this cannot happen in Judaism even though it is based on a Law System.
Jesus said "love your neighbour, don't judge your neighbour, be humble." A prominent Pharisee named Hillel in Second Temple Judaism said the same thing. Christians base their religion around the life, death and resurrection of Jesus and on the emergence of early church communities after his death. Jews base their religion on the Hillel's legacy during the emergence and consolidation of rabbinic Judaism after the catastrophic destruction of the Second Temple.
Here's a comparison:
1. Being Judgmental
Hillel: "judge not your fellow man until you yourself come into his place" (M. Abot 2:5)
Jesus: "do not judge, and you will not be judged" (Luke 6:37)
2. Humility and Greatness
Hillel: "My humility is my exaltation; my exaltation is my humility"
Jesus: "For everyone who exalts himself will be humbled, and he who humbles himself will be exalted." (Luke 14:11)
3. Loving your Neighbour
Hillel: "What is hateful to you do not do to your neighbor; this is the whole Law; the rest is commentary!"
Jesus: "So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets" (Matthew 7:12)
What people often regard as "Jewish law" is actually "Jewish halakha" and the Hebrew word "halakha" means something like "way of walking." Jews think in terms of laws, but I believe they also think in terms of something else -- they think in terms of a path or journey. The arabic word "shariah" has a similar meaning.
It's like an extra layer in Judaism. There is "the Law" but there is also "the way" of following the Law. In John 14:6, Jesus says "I am the way, the truth and the life. Nobody comes to the Father except through me." Many Christians assume that because this is in the Gospel of John, the Gospel of John is in the New Testament Canon and the NT Canon is the sacred text of Christianity that following "the way" means being a Christian.
Jesus doesn't say anything about "the way" being associated with a Gospel of John and the NT Canon and with being a Christian. Consider this:
Matthew 5:17-18: Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. I tell you the truth, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished.
Heaven and earth have not disappeared, so the Law is still in effect. If by saying "I am the way" Jesus is really saying, "do what I say, do what I do" -- to follow the "Hillelite ethics" -- love your neighbour, judge not, be humble -- then Judaism is closer to "the way" then we think. It's because Jewish halakha, I assume, is "the way" to the "the Law." What Jesus taught was probably something similar to that.
Now, consider for a moment that Christianity is largely informed by Saul of Tarsus, whose major contribution was the radical revision of that very same law into one short maxim. Can these two faiths be easily reconciled? What sort of revision would be required to accomplish that reconciliation? Would either group accept that revision? Would the revision even be recognizable to either group?
I think on the Christian side, we would need to revise our understanding of "legalism," "the way" of Jesus and how people can avoid legalism both in a Law System and also in theology.
Even if all these faiths could be reconciled, should they be?
A few questions are in order. 1) what would we aim to achieve by reconciliation and 2) what would be your objections to reconciliation?
I know that a lot of people oppose the idea of "ecumenism" because it dilutes a tradition and compromises its mission/goals. I would agree that many world religions have a mission and that people should not compromise that mission. However, I do not believe that having different missions means there can't be some level of integration and assimilation.
In Romans 11, Paul talks about two olive trees and how branches from the wild olive tree will be grafted into the other. This suggests that there will be some kind of integration and assimilation process in the future. The religion of the cultivated olive tree will retain its identity during that process. Its mission and goals will not be diluted or compromised during that process.