Complex sentence: Catholic worship is highly liturgical, focusing on the Mass or Divine Liturgy in which the sacrament of the Eucharist is celebrated and, the Church teaches, bread and wine become the body and blood of Christ through transubstantiation.
Split into simpler sentences: Catholic worship is highly liturgical, focusing on the Mass or Divine Liturgy during which the sacrament of the Eucharist is celebrated. The Church teaches that bread and wine used during the Mass become the body and blood of Christ through transubstantiation.