Slaves to righteousness "freely" choosing, HD? Slavery is a synonym for bondage, i.e. lack of freedom. So, your quote above is an oxymoron. But I am not surprised that you are tripping all over yourself because your theology leads you right into a corner.
Let me recap what you are saying: God is not the author of evil but He uses it for His purpose. If I entice a man to break my neighbor's window, I am not the author of that crime? Is that what you are saying?
We know that God did not create evil. What God created was all good. Now, since He needs evil for His purpose, as you(and Westminister Confession) claim, He made Lucifer and man so that He could have evil to operate with!
In other words, God needs evil for His plan to work, but He can't make it Himself, so He makes two "fall guys" to take the blame and allows them to become evil. That's pretty evil! But it is also satan's lie that appears "sensible" to some.
First of all, God can accomplish His work without evil, I am sure. Second, God does not need anything. Third, making satan and man so that they can be blamed for evil (which God "needs" to use for His purpose) is outright pagan for the lack of a better word.
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