Romans 3:3-4 — For what if some did not believe? shall their unbelief make the faith of God without effect?
God forbid: yea, let God be true, but every man a liar; as it is written, That thou mightest be justified in thy sayings, and mightest overcome when thou art judged.
Unbelief — head knowledge, following the form without inner heart commitment.
Without effect = deprived of its intended use
God forbid = expression of abhorence
The quotation is from Psalm 51:4 — Against You, You only, I have sinned and done what is evil in Your sight, so that You are justified when You speak and blameless when You judge (NASB).
What Paul is saying in these verses is that the Jews received the Law but didn't believe, but …
There's still an advantage to being a Jew.
God's promises to the Jews still stand.
Unbelieving Jews will still be punished.