Saith thus (ουτως λεγε). Paul personifies "the from faith righteousness" (η εκ πιστεως δικαιοσυνη). A free reproduction from Deuteronomy 30:11-14. Paul takes various phrases from the LXX and uses them for "his inspired conviction and experiences of the gospel" (Denney). He does not quote Moses as saying this or meaning this.

Say not in thy heart (μη ειπηις εν τη καρδια σου). Second aorist active subjunctive with μη like Deuteronomy 8:17. To say in the heart is to think (Matthew 3:9).

That is, to bring Christ down (τουτ' εστιν Χριστον καταγαγειν). Second aorist active infinitive of the common verb καταγω, to bring or lead down. It is dependent on the preceding verb αναβησετα (shall ascend). Τουτ' εστιν (that is) is what is called Midrash or interpretation as in Romans 9:8. It occurs three times here (verses Romans 10:6-8). Paul applies the words of Moses to Christ. There is no need for one to go to heaven to bring Christ down to earth. The Incarnation is already a glorious fact. Today some men scout the idea of the Deity and Incarnation of Christ.