The Church is in its fullness and totality is holy. This is because the profoundness of its identity is that the Church is the body of Christ, the Son of God. With the Father and the Spirit hailed as ‘alone holy’ Christ loved the Church as his Bride and gave himself as his body and endowed her with the gift of the Holy Spirit for the glory of God.
United with Christ, the Church is sanctified by Him; through Him and with him she becomes sanctifying. Thus, The Church is holy because Jesus gives us salvation through the Church.
The church on earth is endowed already with a sanctity that is real though imperfect. In her members we are all sinners in need of redemption. Thus, the church is not holy because we have chosen to associate ourselves with Christ but rather that Christ has chosen to associate himself with you. He has endowed the church with all the means of sanctification through the sacraments, scriptures, the fullness of faith, saints and witnesses as examples, these are ways that Christ is sanctifying us through the the holiness of the church.