"Sanctifying" is simply leading us to live in a God-pleasing or "holy" way consistent with our calling. In fact, a better non-word to replace "sanctify" would be "holi-fy" or "make us holy." But this is widely misunderstood. You don't "live a certain way" to BECOME holy; you are made holy and that's why you live a certain way. In the end, "sanctifying" us has more to do with keeping us alive in the faith we've ALREADY BEEN GIVEN than anything else. By calling us into the faith, the Holy Spirit has already made us "holy." Now He leads us to live life in a manner consistent with our call to be "holy." That is "sanctification."
"But the meaning also says, 'In the same way, He calls, gathers, enlightens, and sanctifies the whole Christian church on earth.' Why the redundancy? Why say the same thing twice?"
Because it's not the same thing! The Holy Spirit calls individuals, and as He calls individuals, He builds up the church as well. But the Church is more than just a "collection of like-minded individuals," right? There are relationships involved in the church! And mutual understanding!
For example: As a Christian in society, you are called to forgive those who sin against you. And that is what the Church does, as well. But apart from the Church, society doesn't do that. (Or if it does, it certainly isn't the "norm.") Society is governed by the Law; the foundation of the Church is the Gospel. And there is nothing "natural" about the Gospel. So the Church must be formed by the Spirit's power, too; not just the individuals in it.
