presuming to celebrate, using
fermented bread and a wooden cup."

_I answer that,_ Two things may be considered touching the matter of
this sacrament, namely, what is necessary, and what is suitable. It
is necessary that the bread be wheaten, without which the sacrament
is not valid, as stated above (A. 3). It is not, however, necessary
for the sacrament that the bread be unleavened or leavened, since it
can be celebrated in either.

But it is suitable that every priest observe the rite of his Church
in the celebration of the sacrament. Now in this matter there are
various customs of the Churches: for, Gregory says: "The Roman Church
offers unleavened bread, because our Lord took flesh without union of
sexes: but the Greek Churches offer leavened bread, because the Word
of the Father was clothed with flesh; as leaven is mixed with the
flour." Hence, as a priest sins by celebrating with fermented bread
in the Latin Church, so a Greek priest celebrating with unfermented
bread in a church of the Greeks would also sin, as perverting the
rite of his Church. Nevertheless the custom of celebrating with
unleavened bread is more reasonable. First, on account of Christ's
institution: for He instituted this sacrament "on the first day of
the Azymes" (Matt. 26:17; Mk. 14:12; Luke 22:7), on which day there
ought to be nothing fermented in the houses of the Jews, as is stated
in Ex. 12:15, 19. Secondly, because bread is properly the sacrament
of Christ's body, which was conceived without corruption, rather than
of His Godhead, as will be seen later (Q. 76, A. 1, ad 1). Thirdly,
because this is more in keeping with the sincerity of the faithful,
which is required in the use of this sacrament, according to 1 Cor.
5:7: "Christ our Pasch is sacrificed: therefore let us feast . . .
with the unleavened bread of sincerity and truth."

However, this custom of the Greeks is not unreasonable both on
account of its signification, to which Gregory refers, and in
detestation of the heresy of the Nazarenes, who mixed up legal
observances with the Gospel.

Reply Obj. 1: As we read in Ex. 12, the paschal solemnity began on
the evening of the fourteenth day of the moon. So, then, after
immolating the Paschal Lamb, Christ instituted this sacrament: hence
this day is said by John to precede the day of the Pasch, while the
other three Evangelists call it "the first day of the Azymes," when
fermented bread was not found in the houses of the Jews, as