St. Ambrose of Optina 1812-1891
Reading spiritual books without instruction, you fear how not to fall into some incorrect thoughts or incorrect opinions. Your fear is well justified. Therefore, if you do not wish to suffer such a spiritual affliction, do not indiscriminately read all manner of new works, even if they are of spiritual content, but are written by those who have not confirmed their teaching by holiness of life; but rather read such works of the Fathers as have been recognized by the Orthodox Church as being fully well-known and doubtlessly edifying and soul-saving.
Source: http://thoughtsintrusive.wordpress.com/2013/04/08/st-ambrose-of-optina/
