We should always be ready whether it is convenient or not to give a clear intelligent and comprehensible answer for the hope that we have in Jesus Christ, unfortunately many who have been led by the Holy Spirit into the revelation of grace as preached by the Apostle Paul are not able to do that, but if you are really interested in knowing why it is GRACE and NOT LAW or even worse a mixture of the two, then you must UNDERSTAND *both* law and grace, that's what gave Paul the advantage over Peter.
Paul was versed in the law and illuminated to grace, where can you find this kind of understanding? I whole heartedly recommend the following book by Martin Zender, "The First Idiot in Heaven" published by Starke and Harmann Inc. You can find both electric and hard copies on Amazon.ca.......this book is a light shining in the darkness, the darkness can not comprehend it but I pray that you will............David L. Ross (Kindle edition currently only $5.00)
https://www.amazon.ca/First-Idiot-Heaven