Father Z explains the differences between how Lutherans and Catholics perceive the Eucharist. Father Z should know, since he was a Lutheran prior to his conversion to the Faith:
As far as the Eucharist is concerned, Catholics believe that, with the consecration by a validly ordained priest (Lutherans do not believe in sacramental ordination that confers an ontological character – rather, every man is his own priest), bread and wine are changed in their substance into the Body and Blood of Christ even though the outward appearance and characteristic accidents of bread and wine remain for our human senses. After this change of substance, trans-substantiation, Christ is truly present in the Eucharistic species, Body, Blood, Soul and Divinity. So long as the outward accidents remain and the species are recognizable as, in their accidents, being bread and wine, they are still the Eucharist and Christ is truly present in them, even in very small quantities of the Eucharistic Body and Blood. When the Eucharistic species are destroyed or significantly altered in their outward accidents, they cease being the Eucharist and Christ is no longer present in them. Furthermore, we Catholic believe that the celebration of the Eucharist represents and renews and makes present again both the Last Supper of the Lord during His Passion as well as the Sacrifice of Cross on Calvary. The celebration of the Eucharist is Christ’s atoning, propitiatory Sacrifice, which, though it occurred at one fixed point in time, is renewed and made present again through the actions of the priest, who acts as alter Christus. Mass is the true, real, renewal of the Sacrifice in an unbloody way that once took place in a bloody way, historically, on Calvary. This is done through the actions and words of the ordained priest.
Lutherans believe that anyone can celebrate the “Lord’s Supper” (some few Lutherans call it “Mass”) though some are called by the community to preside in the central role. The Lord’s Supper is not the Sacrifice renewed. Lutherans do not believe that the substance of bread and wine change, transubstantiation. They think that Christ is present together with the bread and wine for as long as Christ is needed to be there, a kind of “consubstantiation”. (Some Lutherans don’t like that term, but I’m not getting into that fight.) That is to say, that for Christ to be present, there must be institution, distribution and reception. If it is not received, Christ isn’t present. Once no longer needed there for reception, Christ is no longer present and there is left merely bread and wine. They believe Christ is truly present, when required for reception, but not in an enduring way. Luther used the image an iron that is heated and then it cools again: the iron and the heat are there together and then only the iron is there. However, some Lutheran churches are starting to reserve their eucharistic species and even to adore what they reserve, even kneeling outside their eucharistic communion services. An interesting development as they become more “sacramental”. Furthermore, the Lord’s Supper is a memorial merely. It does not renew the Sacrifice of Calvary or the Last Supper, but rather commemorates them. Lutherans believe in a priesthood of all believers. There is no sacramental priesthood or consecration of the Eucharist or sacramental absolution of sins or conferral of confirmation. Matrimony is not a sacrament, nor is anointing. Lutherans have two sacraments, Baptism and “Eucharist”. Their baptism is valid because water is poured on the skin while the Trinitarian form is pronounced. Their “Eucharist” is not the Eucharist. They do not believe it is a sacrament in the sense we do and there is no valid priesthood to confect it, etc. They do not believe, as Catholics do, that sacraments are outward signs instituted by Christ Himself that confer grace. For Lutherans, they are outward signs of realities that are taking place.
Also, I recall when I was younger that, at the Luther Northwestern Seminary in my native place, they would annually have their Lutheran form of “Mass” in Latin. Yes, there are/were such things.
Lutherans, in a way, look on their confession of sins as a sacrament. Luther referred to “penance” as a sacrament in the Large Catechism. Some Lutheran prayerbooks have a rite for penance. But they do not believe that the action and words of the one who hears that confession and pronounces words of forgiveness are sacramentally effective, as when the Catholic priest gives absolution.
This isn’t everything that can be said about differences between Catholic and Lutheran beliefs. Books are written about each aspect I touched on. But this is a start for those who know very little about them.
However, if you are a true Lutheran, you don’t believe what the Catholic Church believes about Eucharist and Priesthood. Not believing what we believe, you cannot receive Communion in the Catholic Church. If you believe what the Catholic Church teaches… then you had better become a Catholic in order to be true to yourself.
I would also say, if you are Catholic but you don’t adhere to Catholic teaching, you are in serious trouble, particularly if you have been properly instructed and you choose against Catholic teachings.
The Second Vatican Council issued a spiritual warning in Lumen gentium 14. Here it is via the Catechism of the Catholic Church when addressing the issue of salvation outside the Church (846ff):
How are we to understand this affirmation, often repeated by the Church Fathers? Re-formulated positively, it means that all salvation comes from Christ the Head through the Church which is his Body:
Basing itself on Scripture and Tradition, the Council teaches that the Church, a pilgrim now on earth, is necessary for salvation: the one Christ is the mediator and the way of salvation; he is present to us in his body which is the Church. He himself explicitly asserted the necessity of faith and Baptism, and thereby affirmed at the same time the necessity of the Church which men enter through Baptism as through a door. Hence they could not be saved who, knowing that the Catholic Church was founded as necessary by God through Christ, would refuse either to enter it or to remain in it(LG 14).
I quoted LG 14 via the CCC to show that what LG 14 contains isn’t obsolete.
If you are not Catholic, but you have come to believe what the Catholic Church teaches, and you refuse to enter the Church by your own will (not because you are afraid, etc.), you are in serious peril for your eternal soul.