For what if some did not believe? shall their unbelief make the faith of God without effect?
Note 4 at Ro 3:3: Paul was using the phrase "the faith of God" in this verse interchangeably with the phrase "the oracles of God" in Ro 3:2. He was saying that the unbelief of the Jews did not make the Word of God, or the promises of God, without effect. Therefore, the Word of God is the faith of God. No wonder faith comes by hearing God's Word: God's Word contains His faith.
Note 5 at Ro 3:3: Paul was asking the question, "If some of the Jews did not believe God's Word, does that make God's Word of no effect?" The answer to this question is a resounding "no" (Ro 3:4). However, Mr 7:13 says we make the Word of God of none effect through our traditions. What's the harmony between these two verses?
People can make God's Word of no effect in their personal lives. Heb 4:2 says God's Word will not profit people unless they mix it with faith. So the Word will not profit anyone who doesn't believe it, but God's Word itself doesn't lose any power. That is what Paul was stating here. The unbelief of the Jewish nation as a whole did not void the promises of God concerning salvation through a savior. The promises of salvation were of no effect to the individuals who rejected Jesus, but to those who will put their faith in Jesus as Messiah, the Word of God still has its power to save.