Romans 1:17 — For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith.
For therein — refers back to the gospel mentioned in verse 16, the revelation of the righteousness of God. Righteousness is God's holiness toward us. Jesus Christ is God's righteousness. He is how God treats us.
Revealed — continuous, is being revealed
From faith to faith — I've often heard that this is referring to faith being passed on from one believer to another, but I think I've now hit on a better explanation — from faith (subjective — Christ's faithfulness) to faith (objective — our response in faith to Him)
As it is written, the just shall live by faith — from Habakkuk 2:4 — Behold, as for the proud one, his soul is not right within him; but the righteous [just, justified] will live by his faith.