Why God Cannot Sin

"In God purity of being is before purity of willing or doing. God does not will the good because it is good, nor is the good good because God wills it; if such were the case, there would be a good above God or the good would be arbitrary or changeable. Instead, God's will is the expression of his nature, which is holy."

--- Henry C. Thiessen, Lectures in Systematic Theology, p. 84