Since someone else has answered on the basis of what is written in Mark's account alone, I thought it might also be useful to see what is in the whole of the Gospels.
Mark's account is paralleled in the Gospels of Matthew and John. In the Byzantine lectionaries, the relevant passages are:
Mark 9:33–10:1
And he came to Capernaum: and being in the house he asked them, What was it that ye disputed among yourselves by the way? But they held
their peace: for by the way they had disputed among themselves, who
should be the greatest. And he sat down, and called the twelve, and
saith unto them, If any man desire to be first, the same shall be last
of all, and servant of all. And he took a child, and set him in the
midst of them: and when he had taken him in his arms, he said unto
them, Whosoever shall receive one of such children in my name,
receiveth me: and whosoever shall receive me, receiveth not me, but
him that sent me. And John answered him, saying, Master, we saw one
casting out devils in thy name, and he followeth not us: and we forbad
him, because he followeth not us. But Jesus said, Forbid him not: for
there is no man which shall do a miracle in my name, that can lightly
speak evil of me. For he that is not against us is on our part. For
whosoever shall give you a cup of water to drink in my name, because
ye belong to Christ, verily I say unto you, he shall not lose his
reward.
And whosoever shall offend one of these little ones that believe in me, it is better for him that a millstone were hanged about his neck,
and he were cast into the sea. And if thy hand offend thee, cut it
off: it is better for thee to enter into life maimed, than having two
hands to go into hell, into the fire that never shall be quenched:
Where their worm dieth not, and the fire is not quenched. And if thy
foot offend thee, cut it off: it is better for thee to enter halt into
life, than having two feet to be cast into hell, into the fire that
never shall be quenched: Where their worm dieth not, and the fire is
not quenched. And if thine eye offend thee, pluck it out: it is better
for thee to enter into the kingdom of God with one eye, than having
two eyes to be cast into hell fire: Where their worm dieth not, and
the fire is not quenched. For every one shall be salted with fire, and
every sacrifice shall be salted with salt. Salt is good: but if the
salt have lost his saltness, wherewith will ye season it? Have salt in
yourselves, and have peace one with another. And he arose from thence,
and cometh into the coasts of Judaea by the farther side of Jordan:
and the people resort unto him again; and, as he was wont, he taught
them again.
Matthew 18:1–11
At the same time came the disciples unto Jesus, saying, Who is the greatest in the kingdom of heaven? And Jesus called a little child
unto him, and set him in the midst of them, And said, Verily I say
unto you, Except ye be converted, and become as little children, ye
shall not enter into the kingdom of heaven. Whosoever therefore shall
humble himself as this little child, the same is greatest in the
kingdom of heaven. And whoso shall receive one such little child in my
name receiveth me. But whoso shall offend one of these little ones
which believe in me, it were better for him that a millstone were
hanged about his neck, and that he were drowned in the depth of the
sea. Woe unto the world because of offences! for it must needs be that
offences come; but woe to that man by whom the offence cometh!
Wherefore if thy hand or thy foot offend thee, cut them off, and cast
them from thee: it is better for thee to enter into life halt or
maimed, rather than having two hands or two feet to be cast into
everlasting fire. And if thine eye offend thee, pluck it out, and cast
it from thee: it is better for thee to enter into life with one eye,
rather than having two eyes to be cast into hell fire. Take heed that
ye despise not one of these little ones; for I say unto you, That in
heaven their angels do always behold the face of my Father which is in
heaven. For the Son of man is come to save that which was lost.
Luke 16:15–18, 17:1-4
And he said unto them, Ye are they which justify yourselves before men; but God knoweth your hearts: for that which is highly esteemed
among men is abomination in the sight of God. The law and the prophets
were until John: since that time the kingdom of God is preached, and
every man presseth into it. And it is easier for heaven and earth to
pass, than one tittle of the law to fail. Whosoever putteth away his
wife, and marrieth another, committeth adultery: and whosoever
marrieth her that is put away from her husband committeth adultery.
Then said he unto the disciples, It is impossible but that offences
will come: but woe unto him, through whom they come! It were better
for him that a millstone were hanged about his neck, and he cast into
the sea, than that he should offend one of these little ones. Take
heed to yourselves: If thy brother trespass against thee, rebuke him;
and if he repent, forgive him. And if he trespass against thee seven
times in a day, and seven times in a day turn again to thee, saying, I
repent; thou shalt forgive him.
These three accounts are harmonized to an extent in the Diatessaron of Tatian, a 2nd century Syriac harmony of the Gospels:
8 And in that hour came the disciples to Jesus, and said unto him,
Who, thinkest thou, is greater in the kingdom of heaven? [Mt 18:1]
9 And Jesus knew the thought of their heart, and called a child, and
set him in the midst, and took him in his arms, and said unto them,
[Lk 9:47, Mk 9:36]
10 Verily I say unto you, If ye do not return, and become as
children, ye shall not enter the kingdom of heaven. [Mt 18:3]
11 Every one that shall receive in my name such as this child hath
received me: and whosoever receiveth me receiveth not me, but him that
sent me. [Lk 9:48, Mk 9:37]
12 And he who is little in your company, the same shall be great.
[Lk 9:48]
13 But whosoever shall injure one of these little ones that believe
in me, it were better for him that a great millstone should be hanged
about his neck, and he should be drowned in the depths of the sea.
[Mt 18:6]
14 John answered and said, Our Master, we saw one casting out devils
in thy name; and we prevented him, because he followed not thee with
us. [Lk 9:49]
15 Jesus said unto them, Prevent him not; for no man doeth powers in
my name, and can hasten to speak evil of me. [Mk 9:39]
16 Every one who is not in opposition to you is with you. [Lk 9:50]
17 Woe unto the world because of trials!5 but woe unto that man by
whose hand the trials come! [Mt 18:7]
18 If thy hand or thy foot injure thee, cut it off, and cast it from
thee; for it is better for thee to enter into life being halt or
maimed, and not that thou shouldest have two hands or two feet, and
fall into the hell of fire that burneth for ever; [Mt 18:8]
19 where their worm dieth not, and their fire is not quenched. [Mt
9:44]
20 And if thine eye seduce thee, pluck it out, and cast it from
thee; [Mt 18:9]
21 for it is better for thee to enter the kingdom of God with one
eye, than that thou shouldest have two eyes, and fall into the fire of
Gehenna; [Mt 9:47]
22 where their worm dieth not, and their fire is not quenched. [Mt
9:48]
23 Every one shall be salted with fire, and every sacrifice shall be
salted with salt. [Mk 9:49]
24 How good is salt! but if the salt also be tasteless, wherewith
shall it be salted? [Mk 9:50]
25 It is fit neither for the land nor for dung, but they cast it out.
He that hath ears to hear, let him hear. [Lk 14:34-35]
26 Have ye salt in yourselves, and be in peace one with another. [Mk
9:50]
27 And he arose from thence, and came to the borders of Judæa beyond
Jordan; and there went unto him thither great multitudes, and he
healed them; and he taught them also, according to his custom. [Mk
10:1]
It might be surprising to some that these passages were related consistently and from a very early time by the Church Fathers to schismatics and heretics. The Greek word translated in the King James Version as "offend" and elsewhere as something like "cause to stumble" is σκανδαλίζω - skandalizo - wherein we see our English word "scandal". It might better be translated as "scandalize" or perhaps "lead astray". It is the same word that is translated as "offend" in ... if thine eye offend thee, pluck it out ... (e.g. Mark 9:47).
The earliest citation of any of these passages by a Church Father that I am aware of is in Clement of Rome's Epistle to the Corinthians, written sometime in the latter first century:
Why are there strifes, and tumults, and divisions, and schisms, and
wars among you? Have we not all one God and one Christ? Is there not
one Spirit of grace poured out upon us? And have we not one calling in
Christ? Why do we divide and tear to pieces the members of Christ, and
raise up strife against our own body, and have reached such a height
of madness as to forget that “we are members one of another?” Remember
the words of our Lord Jesus Christ, how He said, “Woe to that man [by
whom offences come]! It were better for him that he had never been
born, than that he should cast a stumbling-block before one of my
elect. Yea, it were better for him that a millstone should be hung
about [his neck], and he should be sunk in the depths of the sea, than
that he should cast a stumbling-block before one of my little ones.
Athanasius also cites Matthew's account in his Defense against the Arians:
Now if the author of it wrote with an ambition of exhibiting his power
of language, such a practice surely is more suitable for other
subjects: in ecclesiastical matters, it is not a display of eloquence
that is needed, but the observance of Apostolic Canons, and an earnest
care not to offend one of the little ones of the Church. For it were
better for a man, according to the word of the Church, that a
millstone were hanged about his neck, and that he were drowned in the
sea, than that he should offend even one of the little ones. But if
such a letter was written, because certain persons have been aggrieved
on account of their meanness of spirit towards one another (for I will
not impute it to all); it were better not to entertain any such
feeling of offence at all, at least not to let the sun go down upon
their vexation; and certainly not to give it room to exhibit itself in
writing.
Other Church Fathers who related "offending the little ones" to schism and heresy include Basil the Great (Letter XLVI), Leo the Great (Letter XXII), and Gregory the Great (Book of Pastoral Rule).
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