Resorted to put the searches with the most terms first:

Google hits for "Why I am a Catholic" - 2,540
Google hits for "Why I am a Christian" - 1,040
Google hits for "Why I am an Atheist" - 502
Google hits for "Why I am an Agnostic" - 444
Google hits for "Why I am a Democrat" - 331
Google hits for "Why I am a Baptist" - 258
Google hits for "Why I am a Republican" - 229
Google hits for "Why I am a pagan" - 179
Google hits for "Why I am a Unitarian" - 149
Google hits for "Why I am a Libertarian" - 145
Google hits for "Why I am an Anarchist" - 135
Google hits for "Why I am a Socialist" - 107
Google hits for "Why I am a loser" - 105
Google hits for "Why I am a Jew" - 71
Google hits for "Why I am a Communist" - 52
Google hits for "Why I am a Lutheran" - 41
Google hits for "Why I am a Buddhist" - 39
Google hits for "Why I am a Muslim" - 39
Google hits for "Why I am a Presbyterian" - 35
Google hits for "Why I am an Episcopalian" - 21
Google hits for "Why I am a dork" - 21
Google hits for "Why I am a Seventh-Day Adventist"- 20
Google hits for "Why I am a Hindu" - 20
Google hits for "Why I am a witch" - 16
Google hits for "Why I am a Satanist" - 16
Google hits for "Why I am a Wiccan" - 15
Google hits for "Why I am a Methodist" - 14
Google hits for "Why I am an Athiest" - 13
Google hits for "Why I am an Anglican" - 10
Google hits for "Why I am a Scientologist" - 10
Google hits for "Why I am a Monarchist" - 9
Google hits for "Why I am an Evangelical" - 5
Google hits for "Why I am a winner" - 5
Google hits for "Why I am a Druid" - 1
Google hits for "Why I am a Setian" - 1