I recently had the opportunity to briefly interact with someone who posed an interesting question regarding Evangelical Calvinism and its contention that there are still elements of historic Protestant and Reformed theology that still stand in need of reformation (semper reformanda) according to the Word of God. To provide some historical perspective on why we as Evangelical Calvinists make this claim, I would like to quote a brief section from Richard Muller’s magisterial study Post-Reformation Reformed Dogmatics. For those who may not be familiar with Richard Muller, suffice it to say that in Reformed circles he is almost universally recognized as one of, if not the foremost contemporary expert on the rise and development of Protestant theology and its relationship to both medieval and modern thought. For this reason, I think it highly instructive to note what Muller states regarding the scope of the Protestant Reformation:
The Reformation, in spite of its substantial contribution to the history of doctrine and the shock it delivered to theology and the church in the sixteenth century, was not an attack upon the whole of medieval theology or upon Christian tradition. The Reformation assaulted a limited spectrum of doctrinal and practical abuses with the intention of reaffirming the values of the historical church catholic. Thus, the mainstream Reformers reconstructed the doctrines of justification and the sacraments and then modified their ideas of the ordo salutis and of the church accordingly; but they did not alter the doctrines of God, creation, providence, and Christ, and they maintained the Augustinian tradition concerning predestination, human nature and sin. The reform of individual doctrines, like justification and the sacraments, occurred within the bounds of a traditional, orthodox, and catholic system which, on the grand scale, remained substantively unaltered. 
Although only a brief sample of Muller’s extensive and detailed argument, I think this paragraph succeeds in capturing one his main theses with regard to the rise and development of Protestant, and more specifically Reformed, theology. Contrary to those who may believe that the Reformation represented a total and radical break with medieval Catholicism, Muller rightly argues that the scope of the Reformers’ project was decidedly more limited. This is not to say that the Reformation did not send shockwaves throughout the church of the sixteenth century, for of course it did. Nevertheless, it is important not to overestimate the Reformation’s impact on all aspects of medieval theology and practice, much of which remained untouched and unchallenged. As Muller avers, Protestant doctrines such as those pertaining to God, creation, providence, Christ, predestination, human nature, and sin remained substantially harmonious with the preceding currents of thought in the medieval church.
For example, the version of the doctrine of predestination that is often associated with Calvin did not originate with him, although his articulation of it certainly bears a unique stamp. Most of the Reformers including Luther and Zwingli were likewise strongly predestinarian. For all of their differences, the Reformers shared with many of their Roman adversaries similar views on predestination that they both had inherited from the same medieval and patristic sources. ‘Unconditional election’, typically though incorrectly perceived as the sole property of Reformed theology, actually boasts a long historical pedigree going back in some form long before the Reformation to theologians such as Gregory of Rimini, Thomas Aquinas, Gottschalk of Orbais, and ultimately Augustine of Hippo.
The lesson to be learned from this, in my view, is that the somewhat limited nature of the scope of the Reformation may suggest the need for us to continue the work of the Reformers in those areas, such as predestination, that they left largely untouched. This is not to say that everything that the Reformers did not modify was necessarily wrong. It is to acknowledge, however, that we all have our blind spots, the Reformers included, and that we cannot therefore simply assume that the all of the doctrines that they inherited from their medieval forebears and more or less simply assumed and passed on should persist in an identical form in subsequent generations. If the Reformers would have us do anything, it would not be to blindly or unthinkingly reiterate what they said but rather to learn from their example how to relentlessly examine and reform all things in light of the Word of God. This is the essence of sola Scriptura, and we neglect it to our peril.
 Muller, R.A., 2003. Post-Reformation reformed dogmatics: the rise and development of reformed orthodoxy; volume 1: prolegomena to theology 2nd ed., Grand Rapids, MI: Baker Academic. p.97