From J.I. Packer's Keep in Step With the Spirit. Packer discusses the need for personal holiness:
Christians are called to oppose the world. But how, in this case, can that be done? Credible opposition to secular ideologies can be shown by speaking and writing, but credible opposition to unholiness can only be shown by holy living (see Eph. 5:3-14). Ecumenical goals for the church are defined nowadays in terms of the quest for social, racial, and economic justice, but it would be far healthier if our first aim was agreed to be personal and relational holiness in every believer's life. Much as the moden West needs the impact of Christian truth, it needs the impact of Christian holiness even more, both to demonstrate that godliness is the true humanness and to keep community life from rotting to destruction. The pursuit of holiness is thus no mere private hobby, nor merely a path for a select few, but a vital element in Christian mission strategy today. The world's greatest need is the personal holiness of Christian people.
There are those who would surely take issue with Packer's comments regarding the quest for social, racial, and economic justice. Some would object: "But my personal holiness is demonstrated in my quest for social, racial, and economic justice." I certainly agree that part of holy living is embracing such things and promoting them.
However, we need to remember that even the unregenerate can seek and promote social, racial, and economic justice. Those things are not the entirety of personal holiness. Even the most infamous pagan can pursue such things. Personal holiness is about much more than actions. It's about allegiance.