Preached by the Gospel, Revealed by the Faith

It's translated: "from faith to faith". The subject is the righteousness won by Christ and found in the Gospel. The first "faith" is the one settled body of truth. The second "faith" is God-given personal faith. Both "from" and "by" indicate movement toward the object, Christ. It's coming to Christ without moving a muscle. It's the motion of the mind, affections and will.