Note 2 at Lk. 1:6: Zacharias and Elisabeth were not righteous (or lit., in right standing with God) through their own goodness (Isa. 64:6; Rom. 3:10), but their faith was accounted to them as righteousness, just as with Noah (Heb. 11:7) and Abraham (Rom. 4:3). Even under the Old Testament, the only way to be in right standing with God was by faith (Hab. 2:4; Gal. 3:11). No one could keep the law perfectly (Rom. 3:23), so faith had to be put in the mercy and forgiveness of God which was illustrated by the Old Testament blood sacrifices. The Jewish nation as a whole lacked the understanding of justification by faith (Rom.10:3-9) and had fallen into the deception that they could be righteous by their keeping the law (Mk. 10:17). You will notice here in verse 6 that the scripture doesn't say that Zacharias and Elisabeth were sinless but rather blameless. Through their faith in the blood sacrifices (which were symbolic of Christ), God put the blame for their sins on the Lamb (Jn. 1:29).