A holy congregation
When we look at the biblical concept of 'church' (the Greek ekklesia), we see that holiness is a characteristic par excellence. In 1 Peter 2:9, the church is described as 'a chosen race, a royal priesthood, a holy nation, a people belonging to Israel'. These four titles of honor, which first applied to Israel, are now applied to the church (see Ex. 19:6; Isa. 43:20,21).
Christians are called by God into the fellowship of His Son, Jesus Christ (1 Corinthians 1:9). In this we see the first meaning of 'holy': set apart for the service of God. Through baptism, as a sign of the covenant, we are incorporated into the Christian church and distinguished from the world (Heidelberg Catechism, question and answer 74).
In addition, 'holy' has a second meaning. God desires our sanctification. It is the prayer of Christ Himself: 'Sanctify them in Your truth' (John 17:17). This means that God asks us, as members of the congregation, to increasingly reflect the image of Christ. Therefore, in both the Old and the New Testament, the call resounds: 'Be holy, for I am holy' (Leviticus 19:2; 1 Peter 1:16).