The KJV is quite misleading in its translation of Rom 3:30. The Greek text of this verse is undisputed so let me quote it:
εἴπερ εἷς ὁ θεός, ὃς δικαιώσει περιτομὴν ἐκ πίστεως καὶ
ἀκροβυστίαν διὰ τῆς ...
The answer of the question is provided in the quoted verse itself:
“But He said this concerning the Spirit, whom the ones believing into
Him were about to receive”.
“the Spirit, whom the ones ...