The following has been compiled from several CSE questions (and comments); here, here, and here as well as Wikipedia and an article from Slate magazine.
At the second council of Nicaea 787, Catholic authorities passed a law stating that every church should have a relic at its altar. Failure to obey resulted in excommunication. The consecration of an altar was accomplished when the altar had within it or under it the 1st degree relics of two different saints, one of which was required to have been martyred.
A first degree relic is the bone, hair, blood, or tears of a saint. Some believe this tradition of relic veneration began with the collection of ashes following the death of Polycarp, who was burned at the stake in 156 AD.
Up until Vatican II a priest could only celebrate the Mass on a properly consecrated altar. Following the Second Vatican Council in the early 1960's (in 1969 to be precise) the church officially laid to rest the 787 ruling at Nicaea by no longer requiring Catholic churches to posses a holy remnant at their altars.
Altars within churches must be of stone and there are many regulations regarding their construction and use. Movable altars are allowable (hence the birth of the portable altar stone) and an altar used outside of a Catholic Church need not be of stone but of any "suitable" material.
1983 Canon law regarding altars and relics is, in part, as follows:
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.
In "The Order of Dedication of a Church and an Altar " it is written:
The tradition of the Roman Liturgy of placing relics of Martyrs or of other Saints under the altar is fittingly to be retained. Nevertheless, the following should be noted: a) Relics for deposition should be of such a size that they can be recognized as parts of human bodies. Hence, enclosing excessively small relics of one or several Saints is to be avoided. b) The greatest care must be taken to determine whether relics intended for deposition are authentic. It is better for an altar to be dedicated without relics than to have relics of doubtful authenticity deposited under it. c) A reliquary must not be placed on the altar or in the table of the altar but under the table of the altar, in a manner suitable to the design of the altar.
Given that
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.
What exactly is the purpose of the altar in a Roman Catholic Mass? Is it necessary or is it just a table? If it's just a table, does that mean the cloth, corporal, cross, and candles are the necessary parts?
