taken up by many of the Church Fathers including Justin Martyr, Irenaeus and Hippolytus of Rome (c. 170-c. 236). The Holy Eucharist was the central act of worship in early Christianity. The liturgy of the synagogues and the ritual of the Jewish temple, both of which were participated in by early Christians, helped shape the form of the early Christian liturgy, which was a dual liturgy of the word and of the Eucharist; this early structure of the liturgy still exists in the Catholic Mass and Eastern Divine Liturgy. The early Christian use of incense in worship seemed first to originate in Christian funeral rites, and was later used during regular worship services. Incense was also used in the Bible to worship God and symbolize prayer, in both the Old Testament and New Testament; one of the three Magi offered Christ frankincense, and in the Book of Revelation, angels and saints appear in Heaven offering incense to God, thus setting a precedent for Christian use of incense in worship. Reformation liturgies Worship as singing underwent great changes for some Christians within the Protestant Reformation. Martin Luther, a music lover, composed hymns that are still sung today, and expected congregations to be active participants in the service, singing along. John Calvin, in Geneva, argued that while instrumental music had its time with the Levites of the Old Testament, it was no longer a proper expression for the church. This was expanded upon by John Knox (see Presbyterian worship); only Psalms were sung, and they were sung a cappella. Furthermore, in the Genevan and Scottish Reformed tradition, man-made hymns are not sung, being seen inferior to the God-inspired psalms of the Bible. The Calvinist Regulative Principle of Worship distinguishes traditional Presbyterian and Reformed churches from the Lutheran or other Protestant churches. Present day Current Christian worship practices are diverse in modern Christianity, with a range of customs and theological views. Three broad groupings can be identified, and whilst some elements are universal, style and content varies greatly due to the history and differing emphases of the various branches of Christianity. In many Christian traditions, regular public worship is complemented by worship in private and small groups, such as meditation, prayer and study. Singing often forms an important part of Christian worship. Common elements While differing considerably in form, the following items characterise the worship of virtually all Christian churches. Meeting on Sunday