"whoever causes one of these little ones who believe in Me to stumble, it would be better for him to have a heavy millstone hung around his neck, and to be drowned in the depth of the sea.
7“Woe to the world because of its stumbling blocks! For it is inevitable that stumbling blocks come; but woe to that man through whom the stumbling block comes!8“If your hand or your foot causes you to stumble, cut it off and throw it from you; it is better for you to enter life crippled or lame, than to have two hands or two feet and be cast into the eternal fire. 9“If your eye causes you to stumble, pluck it out and throw it from you. It is better for you to enter life with one eye, than to have two eyes and be cast into the fiery hell."-Matthew 18:6-9
It is better for a little one who believes in Jesus to cast off the person who causes them to stumble than for them to stumble and go to the eternal fire.
Why is Jesus saying this if it is true that believers in Jesus can not stumble in unbelief and go to the eternal fire?
Context
Matthew 18
At the same time came the disciples unto Jesus, saying, Who is the greatest in the kingdom of heaven?
2 And Jesus called a little child unto him, and set him in the midst of them,
3 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.
4 Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven.
5 And whoso shall receive one such little child in my name receiveth me.
6 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.
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10 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.
11 For the Son of man is come to save that which was lost.
12 How think ye? if a man have an hundred sheep, and one of them be gone astray, doth he not leave the ninety and nine, and goeth into the mountains, and seeketh that which is gone astray?
13 And if so be that he find it, verily I say unto you, he rejoiceth more of that sheep, than of the ninety and nine which went not astray.
14 Even so it is not the will of your Father which is in heaven, that one of these little ones should perish.
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please pay attention to His will on those who are born again
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then to BROTHERS who may cause one of these young ones to stumble (as opposed to the type of man mentioned in your posted verses)
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15 Moreover if thy brother shall trespass against thee, go and tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother.
16 But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established.
17 And if he shall neglect to hear them, tell it unto the church: but if he neglect to hear the church, let him be unto thee as an heathen man and a publican.
18 Verily I say unto you, Whatsoever ye shall bind on earth shall be bound in heaven: and whatsoever ye shall loose on earth shall be loosed in heaven.
19 Again I say unto you, That if two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of my Father which is in heaven.
20 For where two or three are gathered together in my name, there am I in the midst of them.
21 Then came Peter to him, and said, Lord, how oft shall my brother sin against me, and I forgive him? till seven times?
22 Jesus saith unto him, I say not unto thee, Until seven times: but, Until seventy times seven.