Her two major contributions to theology, for which she was given the liturgical honor of being called Doctor of the Church (one of three women who currently have it), are her letters and The Dialogue of Divine Providence, an account of her visions of Christ. From the latter:
No one born passes this life without pain, bodily or mental. Bodily pain My servants bear, but their minds are free, that is, they do not feel the weariness of the pain; for their will is accorded with Mine, and it is the will that gives trouble to man. Pain of mind and of body have those, of whom I have narrated to you, who, in this life, taste the earnest money of hell, as My servants taste the earnest money of eternal life. Do you know what is the special good of the blessed ones? It is having their desire filled with what they desire; wherefore desiring Me, they have Me, and taste Me without any revolt, for they have left the burden of the body, which was a law that opposed the spirit, and came between it and the perfect knowledge of the Truth, preventing it from seeing Me face to face.
From a letter to Sano di Maco:
We have absolute need of the gift of fortitude, for we are besieged by many foes. The world, with its delights and deceits; the devil, with many vexing temptations, who lights upon the lips of men, making them say insulting and critical things, and who often makes us lose our worldly goods—and this he does solely to recall us from devoted charity to our neighbour; the flesh, astir in our own senses, seeking to war against the spirit. Yes, truly, all these foes of ours have besieged us; yet we need feel no servile fear, because they are discomfited through the Blood of the Spotless Lamb. We ought bravely to reply to the world and resist it, disparaging its delights and honours, judging it to have in itself no abiding stability whatever. It shows us long life, with youth a-blossom and great riches; and they are all seen to be vanity, since from life we come to death, from youth to age, from wealth to poverty; and thus we are always running toward the goal of death. Therefore we need to open the eye of the mind, to see how miserable he is who trusts in the world. In this wise one will come to despise and hate what first he loved. To the wiles of the devil we can reply manfully, seeing his weakness; for he can conquer no one who does not wish to be conquered. One can reply to him then with lively faith and hope, and with holy hatred of one's self. For in such hate one will become patient toward every tempting vexation and tribulation of the world, and will bear these things with true patience, from what side soever they come, if one shall hate one's own fleshliness and love to abide on the Cross with Christ crucified.
She was a reformer by nature, trying to heal the Western Schism, make peace among Italian cities, and correct the abuses of the clergy of her day. In her lifetime she corresponded with Popes and kings, and was never afraid to tell them when she thought they were doing something wrong; they often listened. She is, with Francis of Assisi, the patron saint of Italy.
