(yeah, I skipped a day yesterday, so sue me, it was my birthday :p)
Wow... ummm, okay, I know... let me hunt up a pic...
The Holy Bible
How could any Christian not claim that the Bible doesn't have an impact on their life? It has instructions on every aspect of your life, gives guidance on what to do in times of trouble, and leads the way to salvation.
Yes, even though I'm not a strict follower anymore, this has had the biggest impact in my life.
Although... my husband and kids would have to come a close second ;)