In St. Augustine's correspondence to Januarius (Letter 54) he lays out in three categories the things that guide Christian worship of our Lord. The first category is things directly found in scripture, the second is the apostolic tradition, and the third is adiaphora or things indifferent.
First Category: Scripture
"Hold fast this as the fundamental principle in the present discussion, that our Lord Jesus Christ has appointed to us a 'light yoke' and an 'easy burden,' as He declares in the Gospel: in accordance with which He has bound His people under the new dispensation together in fellowship by sacraments, which are in number very few, in observance most easy, and in significance most excellent, as baptism solemnized in the name of the Trinity, the communion of His body and blood, and such other things as are prescribed in the canonical Scriptures..."St. Augustine's first category is where Christians should naturally begin: those things explicitly taught in scripture, chiefly the proper administration of the Sacraments. By extension we also include many more things taught in scripture: proclaiming the scriptures (1 Tim. 4:13), preaching the scriptures (2 Tim. 4:2), taking up an offering (1 Cor. 16:1-4), praising God through music (Psalm 150; Eph. 5:19), and praying (Matt. 21:13) among others.
Second Category: The Apostolic Tradition
"As to those other things which we hold on the authority, not of Scripture, but of tradition, and which are observed throughout the whole world, it may be understood that they are held as approved and instituted either by the apostles themselves, or by plenary Councils, whose authority in the Church is most useful, e.g. the annual commemoration, by special solemnities, of the Lord’s passion, resurrection, and ascension, and of the descent of the Holy Spirit from heaven, and whatever else is in like manner observed by the whole Church wherever it has been established."The second category are traditions held to that are not explicitly taught in scripture. This is called the apostolic tradition. The examples Augustine cites are from the Church Calendar, namely, the celebration of Easter Sunday, Ascension Day, and Pentecost Sunday. Although these events are directly taken from the scriptures, the scriptures do not demand their celebration to be kept as an annual commemoration. Yet, that does not stop Augustine from commanding that they be observed.
Why does St. Augustine expect Christians to keep traditions not explicitly found in scripture? We need look no further than the Apostle Paul's first epistle to the Thessalonians for our answer. Paul writes, "stand firm and hold to the traditions that you were taught by us, either by our spoken word or by our letter" (2 Thess. 2:15). This is exactly what Augustine is endorsing in our first two categories. Christians hold fast to the doctrines and practices explicitly taught in scripture (by the apostle's letter in the New Testament) and to the unwritten tradition of the apostles (practices taught by the apostles but not written down in scripture). Though the apostolic tradition was not written down by the apostles themselves, we know what these traditions are, for they were written down by their successors.
In his wonderful defense of the doctrine of the Trinity, St. Basil, teaching in the 4th century, names those apostolic traditions: making the sign of the cross, praying facing the East, the invocation/epiclesis of the Holy Spirit over the bread and wine in Holy Communion, and blessing the baptismal water among other things. St. Basil concludes "By what written authority do we do this, if not from secret and mystical [apostolic] tradition?" (On the Holy Spirit, 22.66).
We may plainly see that the apostolic tradition, Augustine's second category for Christian worship, has been preserved for us in our historic liturgies. The classical liturgies (what some churches call an "order of worship"), be it Roman, Anglican, or Lutheran among other rites, protect and guard proper Christian worship. The liturgy delivers to us the apostolic tradition of the historic church.
For both St. Augustine and St. Basil the apostolic tradition (liturgy) is not a category we may dispense of. It is not a "take it or leave it" category. It is a category all faithful Christians everywhere are encouraged to keep.
Third Category: Adiaphora
"There are other things, however, which are different in different places and countries: e.g., some fast on Saturday, others do not; some partake daily of the body and blood of Christ, others receive it on stated days: in some places no day passes without the sacrifice being offered; in others it is only on Saturday and the Lord’s day, or it may be only on the Lord’s day. In regard to these and all other variable observances which may be met anywhere, one is at liberty to comply with them or not as he chooses; and there is no better rule for the wise and serious Christian in this matter, than to conform to the practice which he finds prevailing in the Church to which it may be his lot to come. For such a custom, if it is clearly not contrary to the faith nor to sound morality, is to be held as a thing indifferent, and ought to be observed for the sake of fellowship with those among whom we live."This third category of Christian practice is things indifferent, things we may not bind people's conscience to. The theological word for this category is adiaphora. This is the category of individual conscience; this is the "take or leave it" category.
Augustine demonstrates the variety of eucharistic celebrations to make his point. Some Christians take Holy Communion every day, some only twice a week, still others take only on Sunday. What is important for Augustine is not that we bind people's conscience to make them conform to our practice, but that the practice of celebrating Holy Communion is a regular part of the life of the worshipper. We can say the same about fasting and many other Christian practices. Other things that can fall into the adiaphora category are the placement of prayers and creeds in the Lord's Day service. Should we say the Creed before or after the sermon? Should we take up the offering at the beginning or near the end? Augustine would say these things are indifferent -- there is no right or wrong answer so far as they are done in an orderly manner.