+Paul preaches sola fide because it is the very minimum required. The Protestant error is that it claims faith to be all that is required. In light of the above, may I ask what exactly you believe the Protestant term "Sola Fide" means? If your below is the interpretation, then I do not think it means what you think it means. :) But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.(Rom.4:3)
Nothing could be more clear-Sola Fide-Only faith.