As a small footnote to Jeremy's post, it should also be noted that Trevin Wax's claim regarding the Reformers is rather overstated:

Though the Reformers sought to emphasize the assurance we can have because of God's grace in election and salvation, their descendants sometimes undercut the beauty of assurance by stressing the fruit of sanctification more than the fact of justification.

There is some truth to this but unfortunately, it is a lot more complicated than that.  First, the 'Reformers' were not a monolithic phenomenon so claims about what 'they' thought always need to be read against that background.

Second, even if (for the sake of argument) we allow that Luther and Calvin are typical -- and usually it is these two who are primarily in mind when Christians speak of 'the Reformers' -- the situation is complicated.  Luther's understanding of law and gospel certainly left a place - a large place - for introspection and even despair in the ongoing Christian life.   He was no early advocate of radical sonship theology, despite his being used in this way by some Gospel Coalition writers.   If nothing else, the visitation of the late 1520s, the struggles over the catechisms and the debate with Agricola all point both to the complexity of Luther's development and the ongoing importance of what we might today call 'introspection'.  Yes, for Luther this is the work of the law, not gospel -- but it is crucial to understand that the law remains a vital part of the experience of the Christian.

As for Calvin, a focus on the Institutes (or at least certain passages of the Institutes) might well yield a nicely objective assurance.  A study of the sermons -- the place where classroom theology hits the pew, so to speak -- is rather more variegated. 

Moving beyond Luther and Calvin -- to Zwingli, Tyndale, Hooper etc., the picture gets even more complicated and, in some cases, decidedly introspective -- and that before 1550.

Third, we must remember that the Reformation generated new questions.  The fact is that the Reformers pushed for personal assurance against a background of medieval theology where such was simply not an issue.  Reformation theology generated new pastoral questions, questions it was not in its aboriginal form able to answer; that is why later theologians -- for example, the Puritans -- had to speak in different ways, after years of reflecting upon the pastoral impact of Reformation teaching.  They were striving to answer questions which the very theology of the Reformers has raised for the first time.

Finally, on a personal note (and this is not a shot at Trevin Wax but rather at an apparent current trend): perhaps I live in a very different church world to the rest of American Calvinistic evangelicalism -- that would not surprise me at all -- but in the last few months we have had the Puritans whacked for slavery (and I still cannot name a single Westminster Divine who owned a slave - though I can name a few who, in 1662, lost everything through their stand for the truth) and now for introspection.  Yet is it really the case that uncritical appropriation of the Puritans is the, or even a, pressing problem for the church today? Is legalistic introspection really crippling the church? Are there no other, more threatening problems?  Not weakness on Trinitarianism?  Not books advocating sodomy in marriage? Not the new antinomianism?  Not even new Calvinists who are happy to wear sneakers and buy computers made by slave labour in the Majority World?   The last twelve months seem to have thrown up a few more likely candidates for pressing ecclesiastical problems than John Owen, John Bunyan, and Uncle Tom Goodwin and all.