Paul was the apostle to the gentiles. As such, many of his letters involve explaining the character and requirements of 'the God of the Hebrews' to a world steeped in paganism. In this, he often goes contrary to Rabbinic Judaism, but never against Moses.
The letter to the Romans seems to be addressed to a community struggling to incorporate Jews and Gentiles into the Kingdom of God.
In speaking to Jews, Paul admonishes them not to look down on the Gentiles coming to faith, nor to feel superior because of their heritage in the Scriptures.
To the Gentiles, his advice is not to get cocky because they have been admitted to the Kingdom. God is perfectly able to prune them off and restore the Jews.
Paul's effort to reconcile the two camps into a single body would ultimately fail as the Synagogue and the Church went their separate ways.