If you write for a larger audience than family and friends, expect criticism. Not everyone is gonna like the things you say nor the way you say them. Several thoughts come to mind when considering how to deal with criticism.
View every form of criticism as a learning experience.
Critique partners offer constructive suggestions on how to improve our writing. Readers sometimes offer harsh criticism on comment forums and review sites. Regardless the origin or intent of the one delivering criticism, his words can be turned into pure gold, if we view them as a way to grow, both as a writer and as a person. How is this possible?
Pray first then practice humility.
I'll be the first to admit--criticism, whether intended to help or to hurt, can prick the heart and make me wanna kick the dog, if I had one. But if I'm always trying to defend my words or blubbering the injustice of criticism to others, I'm stuck. I'll never grow. And I'm investing precious energy venting rather than creating and tweaking. The best remedy I've found for dealing with the sting of criticism is prayer.
When you feel that stab of pain, take a deep cleansing breath and run to the Lord. Vent with Him, if need be, then ask Him to give you wisdom as you process the reader's words. If you sense the Lord leading you to reply, let humility guide your response. Often, a simple "thank you" for sharing your thoughts is enough to let the reader know you value his input.
We need not address every reader concern, only those God leads us to interact with. Even the harshest comment may have a bit of truth behind it. And often, pain. While we may not be called as therapists, we are called to be sensitive to the needs of others. Hurtful words often come from a hurting heart. At other times, harsh words are the reader's way of struggling for answers. Rarely, is s/he attacking you as a person. The attack may very well be aimed at what you stand for.
If the reader is challenging your faith, this may be a good opportunity to provide more witness. Again, let the Lord be your guide. Sometimes, a quiet prayer for the reader is all that's required. This, too, is an act of humility. And self-control.
But most of all . . . love.
Allow yourself to grow as your reader audience grows. Learn to view every form of criticism as a learning experience fueled by prayer and met with humility, wisdom, self control, and love.