The alters of catholic churches are the relics of human sacrefice, where in aincent time the bodies of the killed where the alter itself.
>The practice of celebrating the Eucharist over the graves of martyrs is probably the origin of the rule that demanded that every altar must contain the relics of martyrs.
>The General Instruction of the Roman Missal states that "the practice of the deposition of relics of Saints, even those not Martyrs, under the altar to be dedicated is fittingly retained.
>Such relics should be of a size sufficient for them to be recognized as parts of human bodies; hence excessively small relics of one or more saints must not be placed beneath the altar.
>In earlier centuries minute portions of relics were inserted into the table of the altar and also into the altar stones that at that time were called movable altars. The cavity into which they were placed was called the sepulchrum (Latin for "tomb"). The relics could be of several saints, but two had to be martyrs until 1906, when the Congregation of Rites decided that it was sufficient to enclose relics of two canonized saints of whom one was a martyr.
>There were precise rules also about where exactly in the altar the relics were to be placed and about the stone cover for the cavity
Isaiah 65
1“I was sought by those who did not ask for Me;
I was found by those who did not seek Me.
I said, ‘Here I am, here I am,’
To a nation that was not called by My name.
2 I have stretched out My hands all day long to a rebellious people,
Who walk in a way that is not good,
According to their own thoughts;
3 A people who provoke Me to anger continually to My face;
Who sacrifice in gardens,
And burn incense on altars of brick;
4 Who sit among the graves,
And spend the night in the tombs;
Who eat swine’s flesh,
And the broth of [a]abominable things is in their vessels;
5 Who say, ‘Keep to yourself,
Do not come near me,
For I am holier than you!’
These are smoke in My nostrils,
A fire that burns all the day.