Christianity, Concisely

Calvin, in the Institutes, 3.2.1:

First, God lays down for us through the law what we should do; if we then fail in any part of it, that dreadful sentence of eternal death which it pronounces will rest upon us. Secondly, it is not only hard, but above our strength and beyond all our abilities, to fulfill the law to the letter; thus, if we look to ourselves only, and ponder what condition we deserve, no trace of good hope will remain; but cast away by God, we shall lie under eternal death. Thirdly, it has been explained that there is but one means of liberation that can rescue us from such miserable calamity: the appearance of Christ the Redeemer, through whose hand the Heavenly Father, pitying us out of his infinite goodness and mercy, willed to help us; if, indeed, with firm faith we embrace this mercy and rest in it with steadfast hope.