The way of salvation is to believe in the Lord Jesus Christ, and that person will be saved (e.g. Acts 16:31). So in that sense, to be saved one must be a Christian. However, is being a Christian sufficient? Surely trusting in Christ for salvation is enough, isn't it?
The question however goes deeper than what it seems to be asking on the surface. Rather, the question can be phrased thus, "Is bearing the name 'Christian' sufficient?" In other words, should we say that the name "Christian" is enough, and thus we should all just be Christians alone?
The problem with that approach is that it is idealistic and cannot work in the real world. The problem is not with the term, but with Man. People appropriate the label "Christian" all the time when they have no actual right to the term (i.e. liberals like John Shelby Spong). Even for those who seem to be "Evangelical," whatever THAT terms means, does the mere term "Christian" suffice for all of the Christian life? I would say not!
Historically, the Stone-Campbell Movement beginning in the early 19th century was one such movement that wanted to unite all believers who are then just known as Christians. In fact, the early Stoneites called themselves "Christians" with no modifiers attached, and their churches "Christian Churches." Barton Stone was of course extremely shaky on the doctrine of the Trinity (to put it as charitably as possible), and to be honest ought not even to be ordained in the Kentucky Presbytery of the PCUSA, if not for the New Side pastors who focus on experience more on doctrinal fidelity. Stone desired greater unity between all denominations, and the tent revivals on the then American frontier (the most famous being the Cane Ridge revival) was the place where Baptists, Methodists and others can mix and mingle and worship together, united by their pursuit of revivals complete with proto-Pentecostal manifestations. After the union with the "Reformers" headed by Alexander Campbell, the Stone-Campbell movement soon split into the northern Disciples of Christ and the southern Churches of Christ around the time of the Civil War. They were split over issues like instrumental music and the presence of mission boards and other denominational agencies for the Stone-Campbell pseudo-denominations, the anti-institutional institutions.
The history of the Stone-Campbell movement only shows that the idea of just being a Christian is not possible. For what does being a "Christian" mean not seen through the narrow scope of salvation, but also about the church and ministry? Before the CMA, there were Barton Stone and Thomas and Alexander Campbell. Their desire of uniting Christians, and in that time they only had to deal with Protestants who had a stronger link to the Reformation past compared to us, could not be fulfilled. Trying to form a umbrella gathering of all Christians, they only succeeded in creating another new denomination, which then became two denominations. If we are to learn from history, shouldn't it evident that "being a Christian" is manifestly insufficient for the Christian life? If Stone and Campbell cannot pull it off, and neither could subsequent restorationist movements, we shouldn't think any of us could succeed where they fail, this side of heaven.
That is why denominations are necessary. The only alternative to denominationalism is the chaos and faux unity that can be found in the present-day Roman Catholic communion (where there is "unity" between liberals and conservatives), or the splintering into groups ("tribes") based around central dogmas, personalities, parachurch organizations and any other sociological factors (e.g. ethnicity). Far better to have some measure of imperfect ecclesiastical unity, than to have its alternatives.
"Being a Christian" is insufficient for the Christian life. Only "being a Christian" creates more problems than it supposedly solves, is unworkable, and thus we should not be a biblicist in this regard. For example, I am not just a "Christian," but a Christian who is a confessional Presbyterian.