The greatest sin is blasphemy against the Holy Spirit. You, as a bible wonk, should know that.
Then again, I remember how some of your biblical positions used to make me cringe, too. Understanding and agreement are usually only a matter of time.
One suggestion: You might want to really drive home the "sovereignty of God" point, early and often.
Carry on! :-)
There is an even lesser sin that had God's people, under God's orders, kill pregnant women and children. In other words abortion is not remotely the worst sin. Idolatry is far worse. So when I see people who are Christless, Godless Idolators, the killing of their own babies is a lesser side effect that God may do anyway. The real issue is man's salvation. There are a lot of people on the pro-life front that forget this. I am careful not to cross that line. Sometimes prolifers mistake that as being hardened to the killing of babies but I try to keep all in a biblical prespective.