What exactly is the falsehood that St. Paul is referring to at Roman 3: 7? We see St. Paul, while discussing about the merits and demerits of believers getting circumcised, posing an open question at Romans 3:7 (NRSVCE):

But if through my falsehood God’s truthfulness abounds to his glory, why am I still being condemned as a sinner?

My question is: According to Catholic Church, what exactly is the falsehood that Paul is referring to at Roman 3:7?
The overarching context is the necessity of the Gospel for salvation.
In Romans 3:5-8,


But if our injustice commend the justice of God, what shall we say? Is God unjust, who executeth wrath?
(I speak according to man.) God forbid: otherwise how shall God judge this world?
For if the truth of God hath more abounded through my lie, unto his glory, why am I also yet judged as a sinner?
And not rather (as we are slandered, and as some affirm that we say) let us do evil, that there may come good? whose damnation is just. ,


St. Paul refutes, with reductio ad absurdum arguments, a 6th objection that

If our unjustness (= sinfulness) serves to make God’s justness (= justice)
stand out the more clearly, why are we still threatened with his wrath, and urged to seek justification? Is God not unjust in punishing such sinfulness?

Verse 7

is taken by most commentators as a further objection, in which case it is but a weakened repetition of v 5.

—The Epistle to the Romans, commentary by A. Theissen, A Catholic Commentary on Holy Scripture PDF p. 2100
cf. also St. Thomas Aquinas's commentary on Romans 3:1-8