It is the Apostle Paul’s custom, when beginning a letter to a specific church, to highlight the fact that they are “saints,” or “Holy people” as the NIV translates it:
To the church of God in Corinth, to those sanctified in Christ Jesus and called to be his holy people, together with all those everywhere who call on the name of our Lord Jesus Christ—their Lord and ours (I Corinthians 1:2).
True believers are “sanctified” in Christ, that is to say, “set apart.” We are different from the world. We were born in sin, but we are made righteous through the perfect sacrifice of Jesus’ blood on the cross. We were called by God to be Holy. Yet there is an aspect of our sanctification that is still an ongoing process. We still sin–which will be evident as we study the book of I Corinthians. So we must devote our entire will towards consciously choosing holiness, which we achieve not through our own power but through God’s power. We must repent of our sins and submit to God.
All true believers are part of the same family of God. That is why the Apostle Paul addresses this letter to “the church of God in Corinth… together with all those everywhere who call on the name of our Lord Jesus Christ.” We are all brothers and sisters in Christ, even though separated by geography.