Romans 3:3-4

³ For what if some did not believe? Will their unbelief make the faithfulness of God without effect?

⁴ Certainly not! Indeed, let God be true but every man a liar. As it is written: “That You may be justified in Your words, and may overcome when You are judged.”

Unbelief — head knowledge, following the form without inner heart commitment.

Without effect = deprived of its intended  use

Certainly not! = expression of abhorrence

The quotation is from Psalm 51:4 — Against You, You only, have I sinned, and done this evil in Your sight—that You may be found just when You speak, and blameless when You judge.

What Paul is saying in these verses is that the Jews received the Law but didn’t believe, but …

1) There’s still an advantage to being a Jew.

2) God’s promises to the Jews still stand.

3) Unbelieving Jews will still be punished.