This section starts with a bang. Our modern society does not care much for absolutes and hard choices--after all, "only a Sith deals in absolutes" was a well-applauded line (despite the irony of that statement being an absolute)--and casting things in shades of gray has been a long-running habit.
But Jesus makes it very, very plain, that anyone who is not with Him is against Him. That is not the world's way. We want to join in for some of the effort, pull out for others.
The Christian life is not so--we are either fully committed to the Lord or we are involved on the other side. This is one of the errors of our modern society. We want to take a little Jesus here, try a little Christianity over there, when all the while God's Word tells us that we have to choose.
This is the challenge of Christian discipleship: to understand that we live by grace. Full stop. Without grace, we have nothing. We can do nothing. Without Christ, we have no grace.
The next step, though, is to respond to grace. I fear too many times we've stopped without responding to the command of God, and that this is the source of so many of our difficulties in life as Christians. We continue to attempt to live as if nothing is expected of us because we are forgiven. Nothing may be required of us, but is that the way to respond to God's grace?