A sin is a disconnection from the supreme holiness, an attack on the oneness of God, beside Whom there is none other.
Every sin is idolatry, an act of rebellion, for with it a person casts off the yoke of the kingdom of heaven and removes himself from the Source of life and holiness.
From this standpoint, there is no difference between a major and minor rebellion, between an insignificant or a grave betrayal.
In regard to the essential condition of rebellion, it makes no difference what the specific deed is.
As for the principle that saving a life overrides certain prohibitions and the circumstances when the law calls for the commission of a transgression so as to escape death–we have said that, in essence, no distinction can be drawn between minor and major sins.
But if that is so, why does saving a life override the Sabbath?
Why should we desecrate the Sabbath or engage in other transgressions if that is necessary to save a life?
Why is a distinction nevertheless to be made concerning certain sins?
For idolatry, forbidden sexual relations, and murder, the law is that one must choose death rather than commit the transgression.
For other sins, the rule is that one should transgress rather than be killed.
Why is that?
This is in accordance with the explanation of our Sages, of blessed memory, that "The Torah declares, 'violate one Sabbath for him, that he may observe many Sabbaths.
Saving someone's life overrides the Sabbath, not because violating the Sabbath is not as important as saving human life but because the Sabbath itself gains, as it were.
This is not a comparison of the worth of one commandment with another, but the calculation of the gain and loss within a specific commandment itself.
In this case, if a person were to die because others did not violate one Sabbath on his behalf, he would not be able to keep many future Sabbaths.
So although this time the holiness of the Sabbath would not have been violated, on the other hand, many future Sabbaths will not be sanctified by him and not because of the relative leniency or gravity of the sins.
The reason that one should commit certain sins in order to save one's life, and, rather than commit certain other sins, choose death, is not that there are minor and major sins.
There is no objective scale for the relative severity of sins; there are only calculations that at times justify committing a certain sin for a certain purpose and at other times do not justify transgressing.
But these calculations have no connection to the essential nature of the sin, for each sin is inherently as serious as idolatry.
–Rabbi Adin Steinsaltz
From Learning from the Tanya, p. 234, by Rabbi Adin Steinsaltz