There are two ways to look at this question. First, what does the Catholic Church teach about who can participate in Mass (communion)? Second, what should a Protestant consider?
First, according to Catholic teaching, there are five requirements for Catholic communion:
1) The individual must be in a “state of grace” (1 Corinthians 11:27–28). One loses the “state of grace” by committing a mortal sin. Examples of mortal sins are abortion (receiving one or participating in one), homosexual acts, sex outside of marriage, deliberate impure thoughts, etc.
2) The person has attended confession since his/her last mortal sin.
3) The communicant must believe in transubstantiation, meaning the bread is transformed into the actual flesh of Christ and the wine is transformed into the actual blood of Christ.
4) The person must observe the Eucharistic fast by abstaining from food and drink for at least one hour prior to communion.
5) The individual must not be under an ecclesiastical censure such as excommunication.
According to these requirements, a non-Catholic would not be allowed to participate in Catholic Mass.
Second, Protestants need to consider what communion represents. Biblically, the purpose of communion is to remember the death of Jesus Christ and the new covenant and to “proclaim” His sacrifice by means of illustration (Matthew 26:28; Luke 22:19; 1 Corinthians 11:26). In a Catholic Church, the purpose is entirely different. When a person receives communion in a Catholic Church, the priest says, “The Body of Christ,” and the communicant responds, “Amen” in agreement. This signifies a belief in transubstantiation. The vast majority of Protestants strongly disagree with the Catholic understanding of the Lord’s Supper, and it would be dishonest to say, “Amen.”
Receiving Communion in a Catholic church would be to state, “I am in communion/agreement with you.” In the Catholic faith, receiving communion aligns a person in belief and practice with the Church’s doctrine. Given the many theological disagreements between Catholicism and Protestantism, non-Catholics should not participate in Catholic communion.
This concept is confusing to some Protestants because many non-Catholic churches practice “open communion”—that is, they welcome all who have received Jesus Christ as Savior to participate in communion with them. In communion, we welcome brothers and sisters in Christ and join together to remember Jesus’ sacrifice for our salvation. The Lord’s Supper thus becomes a symbol of unity among believers.
In conclusion, a non-Catholic should not partake of Catholic Mass for two reasons. We do not meet the requirements set up by the Catholic Church, and we are not in agreement with the Catholic understanding of the Lord’s Supper. Communion should only take place among believers who share common views on communion and salvation.