Saint John Paul the second once said that the "Eucharist is the heart of the Church." This simple statement reminds us, as the Catechism phrases it, that the Eucharist is the "source and summit of the Catholic Faith." It is from this that one can begin to understand the actions at Mass, the practices of our Holy Communion, and the faith itself. As Catholics, we believe that when the Priest says the words of consecration, the Holy Spirit changes the bread and wine into the Body and Blood, Soul and Divinity of our Lord Jesus Christ.
For that reason, we do not allow those who are not Catholic to receive the Eucharist itself. When one goes forward to receive, the minister holds up the host and says, "The Body of Christ." The communicant is then supposed to respond: "Amen." That Amen is a bold statement. It does not merely affirm that you believe Jesus is present in the Eucharist, but that you accept and profess all that the Church teaches. Saint Paul in the Sacred Scriptures says, "Therefore whoever eats the bread or drinks the cup of the Lord unworthily will have to answer for the body and blood of the Lord." (1 Corinthians 11:27). It is for this reason that we do not have an open communion table.
That does not mean that you cannot or do not participate in the Mass. We welcome all people to join us in singing hymns, praying with Scripture, and even come forward for a blessing from the Priest or Deacon at the Mass. We as Catholics believe the Eucharistic Mass to be the highest form of worship. It is in the Mass that the sacrifice of Christ is made present in the present. That explains each of the actions you see from the Congregation during the Mass itself. Each time we sit or stand, something is happening. There is a change in reading, in solemnity, or the atmosphere. The music, the homily, the words of consecration; all guide us to the truth of the moment.
With that in mind, you will often find Catholics dressed in their Sunday best. That varies from person to person, but what we should keep in mind is that our clothing should reflect our belief. If we are being brought to the foot of the cross, to experience God's merciful outpouring of love and mercy, we should dress in our best clothes. While it is true that God is not a "respecter of persons," it is also true that God deserves the very best we have to offer. Please keep that in mind when choosing what to wear.