The following should be read less as an attack on Christianity than as a defense of prax-based religions. Much of what I say could be used by Muslims or even Catholics. It should be taken as a given that those better read in the intricacies of Christian theology should feel free to correct my explanations of how different Christians understand justification.
One of the big surprises for me when I began to seriously study Christianity was the discovery that Christianity is actually a much more difficult and demanding religion than Judaism. Obviously, it is very easy to be a casual Christian. It is not a challenge to go to church for an hour a week and mumble platitudes about loving your neighbor. The picture changes once we start dealing with committed Christians. Catholicism does place serious demands upon the minority of its practitioners who take the Church's teachings as obligatory as opposed to mere suggestions. Where things get really interesting is when you turn to various branches of Protestantism, which is premised on the rejection of any demand for works and instead relies on faith. (The Armenian tradition within can be seen as an attempt to smuggle works back into Protestantism.)
Eliminating works for faith does not make it easy to be an intellectually serious Protestant. On the contrary, not being able to point to ritual practices to demonstrate that you are a good Christian means that you are completely reliant on your ability to gain the precisely correct frame of mind in order to be saved. It seems simple to claim that all you need is to have faith. The problem is that faith, within Protestantism, does not only mean that you believe that Jesus is the Son of God and part of the Trinity. Having faith means that you believe that Jesus dying on the cross and nothing else is the only way you are saved. You cannot even believe that your good deeds play a role in salvation. Jesus is not going to say: that person performed a few meritorious actions and so I will save him as opposed to the really bad people in the world like the thieves crucified next to me. Everyone is completely depraved and unable to perform even the smallest act of righteousness by themselves.
This sets up Martin Luther's contrast between faith and works. If you are going to believe that doing the right actions can save you, there is no end. No matter how much a person minimizes their pleasure, there is always a more extreme form of asceticism. What is really devious about this is that the more one practices asceticism the less one is thinking about God. The logical end of asceticism is for a person to turn themselves into an idol. Can you believe how righteous I am? For Luther, faith and works contradict each other. If you believe in works, even a little bit, then you do not have genuine faith.
Keep in mind that Luther started off as a highly ascetic Augustinian frier before he left Catholicism for marriage, kids, and beer. More important than possibly nailing 95 theses to a church door was Luther's spiritual crisis as a frier. Did being a frier really save him? Did the fact that he had doubts about whether he was saved prove that he did not really believe and was therefore not saved. The human mind has a particular talent for turning in on itself. Do I really love God or do I just using him to get into heaven? Am I subconsciously trying to convince myself that I love God because I know that I could never fool God into believing that I am sincere? Luther may offer an effective counter to asceticism by eliminating works from the process of salvation but he only makes the problem of internal mind games worse.
Luther's position is only going to drive believers into moderate levels of insanity. According to Luther faith in Jesus requires that you hold two conflicting beliefs at the same time. You must believe that you really are a totally depraved sinner incapable of doing any good deed and, at the same time, that it does not matter because Jesus is willing to save you as long as you have faith. You must simultaneously feel guilty for your sins and serenity for your salvation. Of course, how does a person stop himself from thinking, even subconsciously, that if Jesus is so willing to save him perhaps his sins are not so terrible and he can be saved through his own merits? On the other hand, if his sins really are so terrible, perhaps Jesus will not save him unless he earns his forgiveness with good deeds. If you ever step out of this Lutheran box, you lose your salvation and need to start over. The terrifying reality for Lutheranism is that even if you are saved now, you can lose everything in a few minutes with just the wrong thought. Lutheran religious practice is an ongoing exercise of using humility to constantly get back to that proper balance necessary for salvation and hope that you die at the right moment before you lose your faith again.
All of this makes Calvinism look downright healthy by comparison. If you are willing to accept supralapsarian double predestination that God decided before creation who was going to be saved and who was going to be damned, the doctrine of the perseverance of the saints logically follows. This means that, if you are one of the lucky few elect, you cannot possibly lose your salvation and, therefore, do not have to worry. If you are not one of the elect, you also do not need to worry because it will not help. God decided even before you had a chance to sin to send you to Hell for all eternity.
I have recently learned how to descend to new levels of insanity with Protestantism. In his work Fear of God, John Bunyan (of Pilgrim's Progress fame) argues for the existence of two kinds of "ungodly fear of God." The first is that one believes that he is such a terrible sinner that Jesus would never agree to save him. In this, Bunyan's choice of emphasis differs from Luther. With Luther, the primary concern is that a person might believe that they are righteous enough that they do not really need Jesus. It is probably not a coincidence that Luther started off as a frier while Bunyan started off as a lay Christian who liked to have a good time on Sunday.
So far so good. Bunyan's second kind of ungodly fear, though, is that, once you have accepted Jesus and have been saved, you might go back and question if your conversion was sincere and really valid. According to Bunyan, before a person is saved, he certainly needs to believe that he is not saved but after being saved one is not allowed to doubt their salvation. To do so commits the ultimate Protestant crime of not having faith.
Imagine Bunyan's Christian. He realizes that he is a terrible sinner, who needs to accept Jesus as his savior. Christian prays to Jesus to save him, acknowledging that he has no other means of salvation, not even a lifetime of good deeds. Christian realizes that he is saved and rejoices in this knowledge. Five minutes later, though, Christian begins to think to himself: did I really accept Jesus as my only possible savior or, miserable sinner that I am, simply pretend to accept him?
If Christian was not sincere the first time then he is obligated to question his salvation as he is not yet saved. But if he was sincere the first time, he is not allowed to question his salvation and, by doing so, has thrown himself back into the category of being unsaved. This would mean that Christian needs to accept Jesus a third time unless he was right the second time that this first time was not sincere. Clearly, a doctrine designed to stop people from obsessing too much about whether or not they are saved actually makes the problem even worse.
From this perspective, Judaism is remarkably reasonable in that it avoids this agonizing over whether one is saved with the inevitable question of "are you really truly saved." Judaism can do this precisely because it embraces a doctrine of works wholeheartedly. Judaism does not ask "are you saved." Instead, Judaism asks what mitzvah can you do right now. It does not matter if you are righteous or wicked; there are always mitzvot to be performed. From this perspective, asking whether or not you are saved is a question that one has no business asking. It has nothing to do with fulfilling your purpose in this world so it wastes time better spent doing mitzvot. Since Protestantism does not have an endless stream of mitzvot to perform every minute of the day, committed Protestants have no choice but to spend their time thinking about their salvation until they drive themselves up a wall going in circles wondering if they really have faith.
Belief is still important to Judaism but you come to faith by performing mitzvot. They teach you what to believe and grace you with the ability to persevere in your belief. Furthermore, the very nature of mitzvot precludes radical asceticism. For example, you are obligated to eat a good meal on shabbat and are not allowed to fast. You must get married and have children. You are not allowed to practice celibacy. You are not allowed to give more than 20% of your wealth to charity so apostolic poverty is forbidden. Anyone who says otherwise is a heretic and can be ignored
It is Judaism's emphasis on ritual as opposed to theology that allows it to avoid the pitfalls that will send Christians, if they take their religion seriously, into either the insanities of asceticism or of trying to think their way to salvation. Jewish practice both protects it from trying to earn salvation through asceticism or of trying to simply gain the right beliefs ungrounded in deeds. As much as I find Chabad's theology objectionable, they do get one thing right. They do not care what kind of Jew you are. However religious or not religious you are Chabad will tell you that Hashem loves you so why not thank him by doing a mitzvah right now?