At its core, the Catholic Mass is the representation of Jesus Christ and his sacrifice on the cross. The Catholic priest uses Jesus' words at the Last Supper to change bread and wine into Christ's body and blood, which he then offers to God on behalf of all humanity.
Aside from this core part, many of the other elements of the mass have more or less changed over time, whereas others have remained constant. It is traditional for Catholic participants at mass who are free from mortal sin to receive the Eucharist, which Catholics believe to be Jesus' body and blood under the appearances of bread and wine.
As of 2015, the form of the Roman Catholic Mass divides into the Liturgy of the Word and the Liturgy of the Eucharist. The former consists of readings from the Old and New Testaments with a Psalm that is sung or recited as well. It is customary for the priest or deacon to give a homily and lead the congregation in the recitation of the Nicene Creed on Sundays and feast days. The Liturgy of the Eucharist consists especially of the consecration and reception of the Eucharist, and also includes the recitation of the Our Father.