Regular church attendance and formal church membership are important biblical teachings.

The last couple of weeks in the sermon, I have quoted from John Calvin's Institutes, Book 4, which ties all of this together. Here is Calvin describing what the church is and why we need her:


Because it is now our intention to discuss the visible church, let us learn even from the simple title “mother” how useful, indeed necessary, it is that we should know her. For there is no other way to enter into life unless this mother conceive us in her womb, give us birth, nourish us at her breast, and lastly, unless she keep us under her care and guidance until, putting off mortal flesh, we become like the angels (Matt. 22:30). Our weakness does not allow us to be dismissed from her school until we have been pupils all our lives. Furthermore, away from her bosom one cannot hope for any forgiveness of sins or any
salvation....God’s fatherly favor and the especial witness of spiritual life are limited to his flock, so that it is always disastrous to leave the church.

The Lord esteems the communion of his church so highly that he counts as a traitor and apostate from Christianity anyone who arrogantly leaves any Christian society, provided it cherishes the true ministry of Word and sacraments.

First, all safety resides in Christ alone; and then we cannot be separated from Christ without falling awayfrom all hope of safety; but Christ will not and cannot be torn from his church with which he is joined by an indissoluble knot, as the head of the body. Hence, unless we cultivate unity with the faithful, we see that we are cut off from Christ....

Such as forsake the church...wholly alienate themselves from Christ.


Geddes MacGregor summarized Calvin's view of the church this way:


[God] mercifully chooses to speak to us through the Church. The Church is therefore, according to Calvin, a divinely ordained institution, whose purpose is to accomplish among us the work of the risen and exaltedChrist, who, having, instituted certain ordinances, wills that we recognize in them His divine presence. Those who disdain the fare the Church provides when the Gospel is preached and the Sacraments rightly
administered deserve to ‘perish from terrible hunger.’ In keeping with his patristic and medieval heritage, Calvin treats such people as children who despise their own mother’s milk.


Roderick Campbell describes the necessity of the church this way:


A man without a country (a citizen of no nation) would be considered an anachronism in civil society. A professing Christian who is not a member of any Christian body should be just as much a rarity. There are three institutions and three only -- family, church, and state -- that can rightfully claim the allegiance of every living person. He who refuses, or evades, enrollment in the church of Christ is a traitor to Christ as surely as he who refuses or evades duty to the land in which he lives is considered to be an enemy or a rebel. “He that is not with me,” said Jesus, “is against me; and he that gathereth not with me scattereth abroad” (Mt. 12:30).


Obviously, when Calvin talks about leaving the church, he is not talking about moving from one faithful local congregation to another (there can be good reasons for making such a move and it is possible to do so in a way that respects the authority of the local church). Rather, Calvin is talking about forsaking the church altogether. As I said in the sermon, the Christian life is an ecclesial life, a churchly life. To be a Christian is to be a churchman.
Here is something I wrote years ago on church membership that explores these themes a little more. This and this may also be of interest.