Had the teaching about propitiation through faith in Christ's blood been mentioned before Romans 3?
No. According to the context, propitiation through faith in Christ's blood had never been mentioned before Romans 3. It is introduced in Romans 3:25, where God set forth Christ as the propitiation through faith in His blood, marking a new revelation in the age of grace.
Les explains that simply believing Jesus is the Christ, the Son of the Living God, is foundational but not enough to save anyone in this age of grace. He points to Romans 3, starting at verse 23, that all have sinned and come short of God's glory. Verse 24 says believers are justified freely by His grace through the redemption in Christ Jesus. Then the next verse declares God set forth Christ to be a propitiation through faith in His blood. Les emphasizes that this had never been mentioned before, showing the difference the cross makes.
Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;