What does it mean that God was in Christ reconciling the world to Himself?
It means the entire triune God, with all His power, was present in Christ, reconciling the world to Himself. God did not impute people's trespasses to them but provided everything needed to set them free and reconcile them, then committed the word of reconciliation to believers.
Les explains that reconciliation is God's work, not something we accomplish by grabbing people or knocking on doors. Paul says God has given us the ministry of reconciliation: as He gives opportunity, we must be ready to tell those trapped in Satan's web that God has done everything needed to set them free and reconcile them to Himself. In verse 19, the whole triune God, with all His power, was in Christ, reconciling the world to Himself, not imputing their trespasses, and has committed to us the word of reconciliation. Les then asks why God left us here: to be ambassadors—representatives of a government in a strange land.
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.