In light of many changes in regulations over the centuries, what is the purpose of the Roman Catholic altar and is it neccessary for a valid Mass?
The purpose of an altar is to have a noble material upon which a priest may celebrate the the Eucharist.
The Mass celebrated not on an altar is valid, but illicit.
As the Eucharist stands at “the apex” of all Christian prayers, so too the altar is the central feature within our churches be it is where the priest consecrates the bread and wine into the body and blood of Our Lord Jesus Christ.
The use of fine bronze or cast iron, beautifully carved wood, or various stones may all be appropriate. In all cases the Church has a longstanding preference for stone, especially for the mensa or top. Pope Saint Sylvester (314-335), who was the patron of Saint John Lateran, is said to be the first to have made stone altars obligatory. Saint John Chrysostom writes that “this altar is an object of wonder: by nature it is stone, but it is made holy when it receives the body of Christ”.6 It is this mensa that is anointed at the time of dedication, much like Christ’s body was anointed with oil both before and after His death. Because it is a holy object, we also incense the altar. Just as the altar represents Christ, the stone material represents “Christ the rock” and “the stone that the builders rejected which has become the cornerstone” (Ps 118:22).
The question for us is, how can the design of the altar (and by extension, the area around it) indicate its sacredness and the honor it deserves? People are quite intelligent about these things, and an altar that is a simple wooden table or an abstract metal structure will not be approached with the awe and reverence it actually deserves. An abstract or minimalist object may be fine to read a book on, or eat sandwiches off of, but it can never fully hold our attention.
Instead, the design of the altar should be such that it portrays the theological truths we wish to express. And if we have a beautiful altar, then should we not make the sanctuary and the church beautiful as well?
The mensa, or top stone, receives relics of the saints, following the early tradition of celebrating Mass over the tombs of the martyrs. As the Eucharist is a joining of the Church in Christ’s sacrifice, it can be expressed in the body of a holy man or woman who exhibited that sacrifice in their life. That is, the body or relic of the saint is a material reminder of Christ’s one immemorial sacrifice and bodily death. Saint Ambrose explains that in this way “the triumphant victims may occupy the place where Christ is victim: He, however, who suffered for all, upon the altar; they who have been redeemed by His sufferings, beneath the altar”.7 This is why altars have been dedicated or named after saints, the relics of whom are often placed therein. For instance, in Saint Peter’s Basilica each altar has a dedication to the saint or saints buried underneath. It is imperative for us to recover this cult of the saints and martyrs, especially after the twentieth century, which saw the greatest persecution against Christians since Roman times. - The Altar as the Center of the Church
Modern liturgists may claim that certain changes in the liturgy bring us closer to the way the first Christians worshipped. This may be true, but as it has to be pointed out: The early Christians worshipped in the way they did using a table, for example because they were a persecuted minority, forbidden to build places of worship. Once the persecution ended, they built churches which were a fitting setting for the Holy Sacrifice of the Mass, which was offered in an increasingly elaborate rite inspired by the desire to render the greatest possible glory to God, to whom all honor is due. The way one worships in a time of persecution cannot be considered the norm for a time of freedom.
Now let me address your three point in reference to altars:
Altars consecrated by relics were not required for the celebration of the Mass for 700 years and then were required for 1100 years and now are not required again.
There are regulations in place regarding the use of relics as pertaining to altars (which are no longer required for their consecration).
Portable altars were once not allowable but now a suitable table (with or without a relic) may be used always with the use of a cloth, a corporal, a cross, and candles.
Firstly, altars are consecrated with relics placed into the burial chamber to the mensa or top altar stone. If the altar is placed over the tomb of a saint this burial chamber is not required.
One more point, I would like to clarify here is that a relic of a saint, preferably a martyr. In fact, as you pointed out: Canon Law makes this an obligation.
Canon 1236 §1. According to the traditional practice of the Church, the table of a fixed altar is to be of stone, and indeed of a single natural stone. Nevertheless, another worthy and solid material can also be used in the judgment of the conference of bishops. The supports or base, however, can be made of any material. §2. A movable altar can be constructed of any solid material suitable for liturgical use.
Canon 1237 §2. The ancient tradition of placing relics of martyrs or other saints under a fixed altar is to be preserved, according to the norms given in the liturgical books.
Secondly relics are still required in consecrated altars.
Thirdly portable altars were once not allowed, but now a suitable portable altar may be used if it is equipped to receive a actual “portable” altar stone.
Here is an example of a small moveable altar stone which is place into a portable altar. Thus Mass may be celebrated outside of Catholic Churches.
However the way one worships in a time of persecution or war cannot be considered the norm for a time of freedom.
The Early Church used simple wooden table in the homes of the faithful or in the catacombs in times of persecution.
During World War II, military chaplains were granted the privilege of using an Antimension in lieu of a portable altar stone. This was granted as an indult and as such is not a norm to be followed. Think it as an emergency measure in times of war!
During the Second World War, the Holy See granted to military chaplains the privilege of using for the celebration of Mass, instead of the Latin-rite portable altar stone, "a veil which had enclosed, and well fastened, authentic relics." This was later extended to peacetime military activities. Since it was not always possible to obtain a veil with authentic relics, the use of an Eastern-rite antimension was considered an acceptable alternative.
Finally, Paul VI's "Pastorale Munus," a letter issued "motu proprio" (on his own initiative) in November 1963, gave to all local ordinaries of the universal Church (of all rites, Western and Eastern) the faculty to grant, for a just and serious reason, to all priests subject to them, who enjoy the faculty of the portable altar, the faculty of substituting for the portable altar stone the Byzantine or the Latin forms of the antimension.
Another point I would like to make here is about how the celebration of the Eucharist is celebrated by priests the are seriously persecuted for their faith and some are even been imprisoned for simply being a priest or bishop. I have heard how some have said Mass secretly while in Communist prisons simply using the actual ground as their altar. Many made wine using raisins! Faith finds a way!
Example of a portable altar with a portable altar stone in place.(Have Altar – Will Travel)
In the 14th century the portable altarstone become common.
According to the General Instruction of the Roman Missal, it seems that “”the celebration of the Eucharist in a sacred place is to be carried out on an altar; but outside a sacred place, it may be carried out on a suitable table, always with the use of a cloth, a corporal, a cross, and candles.”
The Altar and Its Appointments
The altar on which the Sacrifice of the Cross is made present under sacramental signs is also the table of the Lord to which the People of God is called together to participate in the Mass, as well as the center of the thanksgiving that is accomplished through the Eucharist.
The celebration of the Eucharist in a sacred place is to be carried out on an altar; but outside a sacred place, it may be carried out on a suitable table, always with the use of a cloth, a corporal, a cross, and candles.
It is appropriate to have a fixed altar in every church, since it more clearly and permanently signifies Christ Jesus, the living stone (1 Pt 2:4; cf. Eph 2:20). In other places set aside for sacred celebrations, the altar may be movable.
An altar is called “fixed” if it is attached to the floor so as not to be removeable; otherwise it is called “moveable.”
The altar should be built apart from the wall, in such a way that it is possible to walk around it easily and that Mass can be celebrated at it facing the people, which is desirable wherever possible. The altar should, moreover, be so placed as to be truly the center toward which the attention of the whole congregation of the faithful naturally turns. The altar is usually fixed and is dedicated.
An altar whether fixed or movable is dedicated according to the rite prescribed in the Roman Pontifical; but it is permissible for a movable altar simply to be blessed.
In keeping with the Church’s traditional practice and the altar’s symbolism, the table of a fixed altar is to be of stone and indeed of natural stone. In the dioceses of the United States of America, however, wood which is worthy, solid, and well-crafted may be used, provided that the altar is structurally immobile. The supports or base for upholding the table, however, may be made of any sort of material, provided it is worthy and solid.
A movable altar may be constructed of any noble and solid materials suited to liturgical use, according to the traditions and usages of the different regions.
The practice of placing relics of Saints, even those not Martyrs, under the altar to be dedicated is fittingly retained. Care should be taken, however, to ensure the authenticity of such relics.
In building new churches, it is preferable to erect a single altar which in the gathering of the faithful will signify the one Christ and the one Eucharist of the Church.