That is sin.
"Therefore, brothers, since we have confidence to enter the holy places by the blood of Jesus, by the new and living way that He opened for us through the curtain, that is, through His flesh, and since we have a great high priest over the house of God, let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water. Let us hold fast the confession of our hope without wavering, for He who promised is faithful. And let us consider how to stir up one another to love and good works, not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near.
For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of judgment, and a fury of fire that will consume the adversaries. Anyone who has set aside the law of Moses dies without mercy on the evidence of two or three witnesses. How much worse punishment, do you think, will be deserved by the one who has trampled underfoot the Son of God, and has profaned the blood of the covenant by which He was sanctified, and has outraged the Spirit of grace? For we know Him who said, 'Vengeance is Mine; I will repay.' And again, 'The Lord will judge His people.' It is a fearful thing to fall into the hands of the living God." - Hebrews 10:19-30
Without the assembling together we are not hearing the Word preached.
Without the assembling together we are not entering into the Holy Place, where Christ gives us HIs own body and blood.
Without the assembling together we are not trusting in the assurance of faith we have received in our baptism.
Without the assembling together we are not confessing our sin, and experiencing the joy of our renewal and the forgiveness which is truly ours in Christ.
Without Word and Sacrament, which only happens in the Church, where we are gathered together in communion with one another, in the Spirit, abiding in Christ, under the faithful care of God the Father we are separating ourselves from grace, we are depriving ourselves from having ears to hear, and a heart to receive the things of God. Then our faith withers, our love diminishes, our hope wanes; we become battered on all sides by the woes, the weariness, and the vanities of this world--which brings with it not faith, hope, and love, and the joy of the Spirit; but the constant stresses of this sin-sick world, in all its temptations, in all its griefs, in all its lifelessness.
Here, in Christ, where Christ's Body is, there is life and joy. For the Word is preached, the Sacraments are administered; for where two or three are gathered in Christ's name He is present; where He is present in His Word, where He is present in His Supper, where He is present through the Spirit, where He is praised and lifted up in the praises, the prayers of His sheep. Where He is King and Lord, Shepherd and Master, and also Friend. Even as the disciples who refused children to come to Him were rebuked, so too we are little lambs and children who need to come to the Lord and be blessed by Him.
There is nothing in the entirety of Scripture which provides for a churchless existence for one who claims to know Christ. It is a pitfall that leads to spiritual death. One which anyone, who having experienced the good of Christ, but having not gathered in for an extended period of time can tell you from first hand experience. I am one of them, as one who has, at times, gone far too long without the gathering together, I am starved for the Lord's Supper, parched to receive again His blood, and His flesh, to hear Him speak through His Word. I become a sheep outside of the discipline and the care of the Shepherd's crook, and become inundated with my own self-importance, become estranged from the mercy of God, quenching the Spirit who brings me life and joy and faith.
Even in ancient times when many receded into the desert and the harsh places to retreat from excessive comfort and reinvigorate their faith through hardship, they never were lone rangers. Even the most remote hermit of the desert caves were dependent, and interdependent, with the Body of Christ.
Nobody is, on their own Christ's Body, the hand cannot say to the foot, "I have no need of you". For by one Spirit we have been baptized, and by one Spirit partaken together of the same drink; and it is by the one Spirit that we are one Body. For of many there is one, as wheat gathered from many places is brought together, and turned into flour and baked into a single bread loaf; so is Christ's Body many gathered from many places and brought together and are one loaf, one Body; and partake of the one Cup and one Loaf that is the Supper of Christ,
"The cup of blessing that we bless, is it not participation in the blood of Christ? The bread that we break, is it not participation in the body of Christ? Because there is one bread, we who are many are one body, for we all partake of the one bread. Consider the people of Israel: are not those who eat the sacrifices participants in the altar?" - 1 Corinthians 10:16-18
Life together was never meant to be easy, we are a diverse menagerie of sinful people with diverse backgrounds, with diverse temperments. Which is why we, like sheep, need both care and discipline, and to be constantly called to life together, abiding in Christ and His Word, by the power of the Holy Spirit,
"I appeal to you, brothers, by the name of our Lord Jesus Christ, that all of you agree, and that there be no divisions among you, but that you be united in the same mind and same judgment. For it has been reported to me by Chloe's people that there is quarreling among you, my brothers. WHat I mean is that each one of you says, 'I follow Paul,' or 'I follow Apollos,' or 'I follow Cephas,' or 'I follow Christ.' Is Christ divided? Was Paul crucified for you? Or were you baptized in the name of Paul?" - 1 Corinthians 1:10-13
"Therefore remember that at one time you Gentiles in the flesh, called 'the uncircumcision' by what is called 'the circumcision', which is made in the flesh by hands--remember that you were at that time separated from Christ, alienated from the commonwealth of Israel and strangers to the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you were once far off have been brought near by the blood of Christ. For He Himself is our peace, who has made us both one and has broken down in His flesh the dividng wall of hostility by abolishing the law of commandments expressed in ordinances, that He might create in Himself one new man in place of the two, so making peace, and might reconcile us both to God in one body through the cross, thereby killing hostility. And He came and preached peace to you who were far off and peace to those who were near. For through Him we have access in one Spirit to the Father. So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus Himself being the cornerstone, in whom the whole structure, being joined together, grows into a holy temple in the Lord. In Him you also are being built together into a dwelling place for God by the Spirit." - Ephesians 2:11-22
The very word confess requires more than one. One cannot confess all by themselves, there but be a with (con) in the fessing. It is homologeo, "to speak the same", to be in agreement with another in what is said. And that meaning is retained in the English translation with the word confessing. To confess with our mouth that Jesus is Lord, to hold to the confession of our faith, to confess our sins--these are corporate words, words of gathering, words of people together. Nobody can confess by oneself, so we confess Christ as Lord, we hold to the confession of our faith, we confess our sins. Nobody is an island, when we act as individuals we are acting in concert with each other; we are not lone wolves, we have a pack, and we have One who is Leader of the Pack, Christ. We are members of a Body, with One who is Head of the Body, Christ. We are not lone houses, but a village, and Christ is Chief. It takes a village to care for the village, it takes a pack to care for the weak and young and sick in our midst so that they don't go hungry or die in the cold, it takes a body to move, act, and do the things which the Head commands it to do. So we are members of one another, children of one Father, alive by one Spirit, in the unity of the one and same Jesus Christ our Lord. This is koinonia, communion, fellowship, togetherness in Christ.
-CryptoLutheran