1) You miss out on the preaching and proclamation of God’s Word.
Private study of the Scriptures is important, but it can never be a replacement for the proclamation of the living word within the living body of the Church. Paul called upon Timothy to devote himself “to the public reading of Scripture, to exhortation, and to teaching.” (1 Tim. 4:13) The public proclamation of the Gospel reminds us that the Gospel is not for our ears only, but is a public Gospel - meant for the whole world. When we miss Sunday worship and the proclamation which takes place both in the readings and from the pulpit, we make the claim that Christianity proclaims a private faith. The truth is that in Jesus Christ, the “revelation of the mystery that was kept secret for long ages has now been disclosed.” (Rom. 16:25)
2) You miss out on praying with the whole family of God.
Liturgical prayer isn’t just about our individual prayers being joined to those of the Church. Liturgical prayer isn’t just about prayer that is corporate. The Christian vision for liturgical prayer, especially in the Sunday liturgy, is much more than that - it is the prayer of Christ, who has joined His Church to Himself, who continually offers His beloved bride to the Father through the Holy Spirit. The Church is therefore not merely the living, but the dead as well, joined together in common worship, sharing a heavenly orientation. The liturgy expresses the Church’s proper nature as a temple of the Lord’s presence and glory, the Incarnate Christ making his dwelling among us. (2 Cor. 6:16)
3) You miss out on fellowship with the Lord’s Body and Blood in the Eucharist.
The Christian life is not powered by human strength or earthly food. The Christian life is powered by divine grace - in particular the grace of Jesus himself. The gift of the Eucharist is, simply put, this grace - the gift of fellowship with the body and blood of Jesus, without which it is impossible to be saved. When we miss worship on Sundays, and particularly when we miss the Eucharist, we are showing forth that we believe more in our own abilities and strengths than in the blood of Jesus to cleanse us from all unrighteousness. We become Pelagians - believing that we can muster enough goodness within ourselves to merit eternal life.
4) You miss out on offering your life and labor to God and to the poor.
The offertory is not only that part of the liturgy where we take up an offering but we offer bread and wine, indeed our very selves, to God in worship and praise. God doesn’t need our money and he doesn’t need our offerings. In Scripture, the sacrifices that honor God are the sacrifices of thanksgiving, the offerings which praise God for his wonderful goodness. This is something which is not simply done with our checkbooks, but with our bodies. Thus, the location of our bodies truly matters. When our bodies are joined to the whole church in making a living sacrifice, something holy and wonderful has happened! Paul writes to the Romans: “I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship.” (Rom 12:1)
5) You miss out on resting from sin and evil. You miss out on enjoying the Sabbath rest of God.
Much of life is toil and endless work. This goes with the territory of the Fall. But, ancient Christians understood that they had been rescued from a life of meaningless toil by the Christ’s death on a Friday, His rest on the Sabbath, and His Resurrection on a Sunday, bringing about for them an endless Sabbath rest. We rest from our works as God did from his out of obedience to His command, reminded that there will come a day when all our works shall cease. Worship on the Sabbath reveals the true end of all our labors. If we have labored for our own success and material wealth, that will be revealed, typically in a begrudging, or lazy, or apathetic, attitude toward worship. But, if we have labored for an inheritance with the saints, for an inheritance that endures, that will also be revealed, typically in the very first hours of every week, turning our hearts and minds to the Lord with the Church, so as to consecrate the whole of each week to His mercy.