What then? If some did not believe, their unbelief will not nullify the faithfulness of God, will it?
I remember growing up hearing the story of "Old Yeller". He was this faithful yellow dog who stuck by his family no matter what they did or what happened in their lives. This is the concept of faithfulness that we love to associate with God's faithfulness to us. No matter what we do or how we fail He is always there to pick us up, dust us off, and then tell us to keep going.
But the significant difference is that God is not faithful to us. He is faithful to Himself. He is faithful to His Word. So while His faithfulness is connected to his forgiveness it is also connected to his righteousness. It is the the other aspect that we often forget. There is accountability to those who have been entrusted with His Word who do not walk in His ways.
For a pastor it is not enough to be preaching the Word of God without living the Word of God. It is not enough to point out the weaknesses of others without first seeing the weakness in ourselves. This is what Christ stated concerning the scribes and pharisees in Matthew 23:3 "therefore all that they tell you, do and observe, but do not do according to their deeds; for they say things and do not do them." Every pastor should preach the sermon to themselves first.
God is faithful to His Word. The rest of Romans 3 addresses this vital truth that all are accountable for their rejection of God regardless of their knowledge and lineage. All have to make a decision to live the Word because in God's faithfulness there will be punishment.
This brings hope and it brings fear. It brings hope because we know that God is faithful to His Word which leads to salvation. It brings fear because we know that God is faithful to His Word and there will be judgement for all of sinned. God's faithfulness to His Word is not dependent on our response. Our response will determine how we are impacted by His faithfulness.