What does it mean that God was reconciling the world to Himself in Christ?
It means the Creator God Himself was in Christ on the cross, bringing the world back to Himself after it was lost in Adam, and not imputing people's trespasses to them. Christ had to be the God of creation to taste death for every man and pay the sin debt of every human being.
Les teaches that verse 19 shows God was in Christ as He hung on the cross. It was the Creator of the universe who hung there—the same God of Genesis 1:1 who called everything into being. He had to be that God, or He could never have tasted death for every man or paid the sin debt of every human being. Jesus is the God of creation who took on flesh and went the way of the cross. Paul is saying God was in Christ, reconciling, bringing the world back to Himself after He lost it in Adam, not imputing their trespasses unto them. Les says this means what it says: until a lost person dies and has lost his opportunity to believe the Gospel, his sins are not being held against him.
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.