Jesus replied, "No one who puts his hand to the plow and looks back is fit for service in the kingdom of God." Luke 9:62
This saying has always scared me. It almost seems as if Jesus is saying that if you have any doubt that you can't be a disciple. The context of this saying is the cost of being a disciple and Jesus is telling us that we must let go of the past in order to embrace the future.
Plowing is a great metaphor. Most of us have probably never plowed a row in our lives, so this is a strange metaphor. In plowing, the wise old farmer knows that you pick a spot in the field, like a tree or rock, and set your aim for that mark. It's tempting to look back and see how your plowing; it even seems to make sense. "How can I know if I'm plowing straight if I don't look back?" The problem is that if you look back, even for a moment, you look sight of your mark and your rows become crooked and you even risk plowing under something you've already planted.
Jesus is telling us that in order to be a follower we must let go and never look back. To be a good tiller of the soil is to keep our eyes on Jesus and never look back at our old life. We must count the cost if we are to be useful in the Kingdom of God.