You can't see the wind, you can only see what it does. That truth is found in scripture and a lot of other writings, ancient and modern. To see what the wind is doing, we look at trees and flags to find out how fast and what direction.
In the same way, people ought to be able to look at me and see what God is doing. After all, I'm one of the ones who gets it, who understands what Jesus did for me, who knows exactly what I owe to God's grace. So when John writes, "Dear friends, since God so loved us, we also ought to love one another. No one has ever seen God; but if we love one another, God lives in us and his love is made complete in us." (1 John 4:11-12), I have to think that my love is the visible evidence people around me can see of God.
On the other hand, when I proclaim my faith with my mouth and then use that same mouth for snarky comments, gossipy innuendo or name-calling, what does that show people? My own weak sinfulness, sure, but they probably see it as evidence that I'm a hypocrite and God doesn't really change hearts.
Love is serious business for Christians. It's how I know I've been changed, and it's how my neighbors see God is real.