If this is the case, then first we have to address what we respectively view to be the definitions of “Christian” and “Christianity.” There are some minor disagreements, to be sure; the disagreements between Presbyterianism and Baptists come to mind. However, we’re typically speaking of disagreements on the order of “I disagree with you on what the Bible says”, or “I disagree with you about what should be orthodox”, or “I disagree that Scripture is the only infallible rule for faith and practice.” For the unbeliever, it tends to be along similar lines. The unbeliever will assume their own worldview to critique our own. As a “former Christian”, they will define Christian doctrine as they now see it – or insist that how they now see it is correct – typically without demonstration. Alternatively, they will insist that group _x_ is the true expression of Christian belief (again, sans demonstration), as opposed to group _y_. Unfortunately, it is often the case that they will distort the views of both groups. When dealing with Christian orthodoxy, it must be understood that in a historical sense, Sola Scriptura has been the rule of faith throughout. This is demonstrated throughout the Scripture, the writings of the Early Church Fathers, and the doctrine developed and exposited over that period of time. Secondly, it must be understood that there is 2,000 years of historical theology to back up this claim. Addressing Jehovah’s Witnesses, or Mormons as “Christian” stretches the bounds of credulity to the breaking point. Insistence on holding Protestants to Roman Catholic or Eastern Orthodox dogma is similarly a strikingly incredulous claim. When making claims as to what is “Christian”, the putative believer (or “former believer”) needs to be cognizant of the history of the church, and her doctrines; otherwise, they are simply offering a straw man up to the blaze. For those who call themselves believers who disagree, keep in mind where your disagreement lies. If it is a disagreement of a second order, such as ecclesiology, this is non-problematic, for the most part. There are places where this disagreement will have repercussions, as is necessarily the case, but if we are in agreement on the major elements on the faith, such as the “Solas” of the Reformation, the nature of the Gospel, or theology proper, there is no fundamental problem involved. If your disagreement is on one of the major topics of orthodoxy, the issue is more serious, and we will be at odds on many, many points; some of which will be concerning whether you have the right to call yourself “Christian.” In any case, please keep these points in mind.