What does it mean that God was in Christ reconciling the world?
It means the whole triune God, with all His power, was in Christ, reconciling the world to Himself and not imputing their trespasses unto them. God has done everything needed to reconcile the world, and He has committed the ministry and word of reconciliation to believers.
Les explains that verse 19 says the whole triune God, with all His power, was in Christ, reconciling the world unto Himself, not imputing their trespasses unto them. Since God has done everything necessary to set a person free and reconcile them to Himself, He has given believers the ministry of reconciliation. As God gives opportunity, believers are to tell people wrapped in Satan's web that God has done everything needed. God has left believers here as ambassadors—representatives of a government in a strange land—so they can carry the word of reconciliation to the world.
To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation.