This statement is the first suggestion that the roots of the tree are inadequate in any way. The Lord explicitly states that the roots are good (v. 36) but simply acknowledged that they “profit nothing.”
The Abrahamic covenant is good, as is the message of the gospel delivered to all of the ancient prophets. Nevertheless, at this point in time, the fruit of the tree is not “profitable.” In spiritual terms, the gospel is available to human beings, but it is not producing power to salvation. The root may be good, but the ultimate goal of the Lord of the vineyard remains unfulfilled. These forms of the gospel do not have the power to exalt.