The Summa Theologica by Saint Thomas Aquinas

PROLOGUE

Forasmuch as our Saviour the Lord Jesus Christ, in order to “save His people from their sins” (Mat. 1:21), as the angel announced, showed unto us in His own Person the way of truth, whereby we may attain to the bliss of eternal life by rising again, it is necessary, in order to complete the work of theology, that after considering the last end of human life, and the virtues and vices, there should follow the consideration of the Saviour of all, and of the benefits bestowed by Him on the human race.

Concerning this we must consider (1) the Saviour Himself; (2) the sacraments by which we attain to our salvation; (3) the end of immortal life to which we attain by the resurrection.

Concerning the first, a double consideration occurs: the first, about the mystery of the Incarnation itself, whereby God was made man for our salvation; the second, about such things as were done and suffered by our Saviour—i.e. God incarnate.