This is a delicate question, as it exposes one of the fundamental differences between the Christian outlook and the Jewish one.
As far as I understand the Christian concept, it runs something like this: Ever since the expulsion of the first man and woman from Eden, the world has been “fallen” and all subsequent human beings have been born inherently sinful, guilty from the moment of birth as a result of the first man’s disobedience of G-d’s commandment not to eat of the ‘Etz haDa‘ath Tov vaRa — usually translated “the Tree of the Knowledge of Good and Evil.” The only way to escape condemnation and eternal damnation for this inherently sinful nature is to accept the blood sacrifice of the Christian savior. In this fashion one can gain salvation, purely as an act of Divine grace, without dessert or merit on the part of the human being.
To my mind, there is a basic flaw in this reasoning. Christians often picture Jews as believing in a dour, harshly judgmental “Old Testament” G-d, despite dozens of statements in Tanach (the proper name for what Christians call the “Old Testament”) expressing G-d’s love of Israel and indeed, all humanity. They juxtapose this to the “loving G-d” of the “New Testament.” This invites the question: What about the billions of human beings who were born and lived their lives without knowing anything whatsoever about the Romans’ execution of a carpenter in Judaea some 2,000 years ago? If G-d is indeed rachum vechanun, “merciful and gracious,” as the Bible asserts (cf. Exodus XXXIV ,6), how could He allow such a state of affairs to go on?
This view of an inherently evil world is often expressed in almost Manichaean terms, with the world dominated by an evil being — “the Devil,” “Satan,” a “fallen angel” — in opposition to G-d.
So what, in fact, do Jews believe?
Consider the terms tov and ra, conventionally translated, as I wrote before, as “good” and “evil.” At every stage of the world’s creation, G-d pronounced it tov before proceeding to the next stage. On the creation of mankind, He pronounced it tov me’od (“very good”), and there is no indication thereafter that He changed his mind.
Ra does not actually mean “evil” in the English sense of the word. Some glimmering of its actual meaning can be ascertained from some of the other ways that the root is used. For instance, in Psalms II, 9 King David beseeches G-d to deal with his enemies: Tero‘em beshevet barzel (“You should smash them with an iron rod”), or in Isaiah XXIV, 19 the prophet begins his description of an earthquake: Ra’o hithro‘a‘a ha’aretz (“the Earth is completely shaken”). From these, we can see that it means something like “unstable, broken, dysfunctional” and therefore “bad.”
Human beings come into this world innocent of anything, but possessed of a capacity for good (commonly termed the yetzer hatov) as well as a destructive capacity, commonly termed the yetzer hara. The yetzer hara presents all the physical urges, the needs and wants, of the physical body which, like everything else in the physical realm, is subject to entropy — that is, it wears out and falls apart. But he is also provided with a soul, whose highest purpose is to control those urges and channel them into positive actions.
To this end, children are provided with parents and other mentors, whose job it is to teach them right from wrong and self-control, so that his soul is capable of taking charge and leading a proper, sanctified life. Until that moment when he is capable of taking over, any “sins” that the child commits are the responsibility of the parent.
So when does a Jewish individual begin to sin? At the age of bar or bath mitzva. These terms mean “son or daughter of the commandments” because on reaching that age, they become subject to the 613 commandments in the Torah, and their parents are no longer responsible for their actions. This landmark occurs when a boy is 13 years old and a girl is 12. One of the most emotional moments of the bar mitzva ceremony comes when the boy’s father pronounces the blessing, baruch sheptarani me‘onsho shel ze (“Blessed is He who has exempted me from this one’s punishment”).
What is the Jewish concept of the satan? Well, we agree with the Christians that he is a mal’ach, conventionally translated “angel,” but there’s nothing “fallen” about him. He works for the same Divine Boss as all the other mal’achim. Think of the satan (the word means “adversary”) as the proctor of an exam. The proctor isn’t actively rooting for you to fail the test; to the contrary, he wants you to pass. But he administers a tough test, to be certain that it tests all your capabilities and that you’ve mastered the material, i.e. the life lessons available from one’s parents and other mentors. If you manage to pass the test, no one is happier than the satan.
Of course, to be constantly consciously aware of one’s actions and to control and channel the yetzer hara requires arduous, exhausting effort; most of us stumble somewhere on the path, which is what the term usually translated “sin,” chet, actually means: “to miss a mark or a target.” For this, there is the process called teshuva, roughly “repentance,” literally “return” to the straight and narrow after having erred and strayed from the path.
Because G-d truly is rachum vechanun, erech apayim verav chesed ve’emeth, notzer chesed la’alafim, mose ‘avon vafesha vachata’a vnaqe (“merciful and gracious, long-suffering, great of kindness and trust, keeping kindness for the thousands, bearing iniquity and transgression and sin, and cleansing.” Exodus ibid.).