There are at least three reasons for this.
The first is that the Church’s worship is unearthly. It is for those who are already citizens of the city of the living God. The writer of Hebrews says that in our worship,
…you (2nd person plural) have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, to an innumerable company of angels, to the general assembly and church of the firstborn who are registered in heaven, to God the Judge of all, to the spirits of just men made perfect, to Jesus the Mediator of the new covenant, and to the blood of sprinkling that speaks better things than that of Abel (Hebrews 12.22-24).Though our physical eyes don’t see this, it is the reality of what is happening in worship, by the work of the Holy Spirit.. To put it in Paul’s words (Ephesians 2.18), through Jesus Christ we have access in the Holy Spirit to God.
Secondly, this new citizenship necessarily requires a new language. Augustine phrased it as, In dominico eloquio (“The Lord’s style of language”). Robert Louis Wilken points out that early on Augustine “recognized that if he were to enter the Church he would have to learn this new tongue, hear it spoken, grow accustomed to its sounds, read the books that use it, learn its idioms, and finally speak it himself. He had to embark on a journey to acquaint himself with the mores of a new country. Becoming a Christian meant entering a strange and often alien world.”1 And as we saw above, worship is entering another “world”.
Finally, the assembly is a foretaste, and a breaking in, of the great day when Christ returns and judges the living and the dead, divides sheep from goats. As the Psalmist puts it in Psalm 1: “The ungodly shall not stand in the judgment, nor sinners in the congregation of the righteous.” The parallel between judgment and congregation is, or ought to be, stunning!
Therefore, Christian worship is meant to be divisive, for the purpose of raising the dead and delivering people “from the dominion of darkness, and transferring” them “into the kingdom of the Son of” God’s “love” (Colossians 1.13). Some people who enter the assembly are meant to feel terribly uncomfortable - so that they may turn to the Lord Jesus now and be rescued from the coming judgment now; and some people who come are meant to be comforted, encouraged, and made holier as they find “that there is therefore now no condemnation for those who are in Christ Jesus.”
1. Robert Louis Wilken, “The Church’s Way of Speaking”, First Things, August/September 2005, Online: http://www.firstthings.com/article/2007/01/the-church8217s-way-of-speaking-24. The emphasis is mine.