The writer of Hebrews encourages us in Hebrews 10:25 that we are to not forsake "the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.
Martin Luther said of this, “Membership in a confessing body is fundamental to the faithful Christian life. Failure to do so defies the explicit warning not to forsake "our assembling together." His understanding of this prompted Martin Luther to say, "Apart from the church, salvation is impossible." Not that the church provides salvation; God does. But because the "saved" one can't fulfill what it means to be a Christian apart from the church, membership becomes the indispensable mark of salvation.”
Brother Luther and I are in complete agreement.