Saint Catherine of Siena (1347-1380), one of the most brilliant theological minds of her day, and one of three women recognized as Doctors (Teachers) of the Church, was called to listen to the Lord as a young girl.  The 24th of 25 children, she started having mystical experiences when she was only 6, seeing guardian angels as clearly as the people they protected.

She became a Dominican tertiary when she was 16, and continued to have visions of Christ, Mary, and the saints.  Having learned to read at 17, she wrote profusely in letters and lessons of accounts of her visions, mystical experiences, and of promoting peace.  She became, in her twenties, a called-upon peace negotiator among warring states and cities.  She stood in the middle of the political, economic and religious movements of the time and left her mark upon all of them as a woman of extraordinary wisdom, clear intellect, strength of spirit, loving heart, and with a theological genius recorded in her writings.  She persuaded the Pope of the time to return from exile in France back to Rome and she advised the Papacy on its work and its ways, recommending turning from power and destruction to love and forgiveness.  St. Catherine’s letters, and a treatise called The Dialogue, are considered among the most profound, mystical writings in the history of the Catholic Church.
  
For more information about St. Catherine, please check out these webpages.
http://www.cin.org/saints/cathsiena.html
http://www.newadvent.org/cathen/03447a.htm
http://en.wikipedia.org/wiki/Catherine_of_Siena