JI Packer states with regards to Luther’s Bondage of the Will:
“Here was the crucial issue: whether God is the author, not merely of justification, but also of faith; whether in the last analysis, Christianity is a religion of utter reliance on God for salvation and all things necessary to it, or of self-reliance and self-effort. ‘Justification by faith only’ is a truth that needs interpretation. The principle of sola fide is not rightly understood till it is seen as anchored in the broader principle of sola gratia. What is the source and status of faith? Is it the God-given means whereby the God-given justification is received, or is it a condition of justification which it is left to man to fulfil? Is it a part of Gods gift of salvation, or is it man’s own contribution to salvation? Is our salvation wholly of God or does it ultimately depend on something that we do for ourselves?”
Martin Luther, On the Bondage of the Will (Revell: USA 1957) pg. 59. (JI Packer’s quote from the introduction to Luther’s work).