In the New Testament, we are exhorted to avoid the unbelief of the children of Israel.

How can we know of the authenticity of scriptures?  Here is a link from a well-known expert.

Why I Believe The New Testament Is Historically Reliable

Hebrews 3:6 (KJV) But Christ as a son over his own house; whose house are we, if we hold fast the confidence and the rejoicing of the hope firm unto the end.

7 Wherefore (as the Holy Ghost saith, To day if ye will hear his voice,

8 Harden not your hearts, as in the provocation, in the day of temptation in the wilderness:

9 When your fathers tempted me, proved me, and saw my works forty years.

10 Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways.

11 So I sware in my wrath, They shall not enter into my rest.)

12 Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.

13 But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.

14 For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;

15 While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.

16 For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.

17 But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?

18 And to whom sware he that they should not enter into his rest, but to them that believed not?

19 So we see that they could not enter in because of unbelief.

Hebrews 4:4 Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it.

2 For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it.

3 For we which have believed do enter into rest, as he said, As I have sworn in my wrath, if they shall enter into my rest: although the works were finished from the foundation of the world.

4 For he spake in a certain place of the seventh day on this wise, And God did rest the seventh day from all his works.

5 And in this place again, If they shall enter into my rest.

6 Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief:

7 Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts.

8 For if Jesus had given them rest, then would he not afterward have spoken of another day.

9 There remaineth therefore a rest to the people of God.

10 For he that is entered into his rest, he also hath ceased from his own works, as God did from his.

11 Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.

12 For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.

13 Neither is there any creature that is not manifest in his sight: but all things are naked and opened unto the eyes of him with whom we have to do.

14 Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.

15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.

16 Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need.

How did they indicate their unbelief?  It was through their words.  It was through murmuring and complaining.

Without going into details of the history of this passage, which you can learn by reading the Old Testament itself, we come to the place where the generation that had viewed the mighty works of the Lord, the plagues sent to Egypt, the Passover, the deliverance from Egypt, the Ten commandments at Mt. Sinai, the manna in the wilderness, the water out of the rock, the miracles of deliverance and preservation by the Lord God, were judged for their murmuring, complaining, and words.

Numbers 14:1   And all the congregation lifted up their voice, and cried; and the people wept that night.

2 And all the children of Israel murmured against Moses and against Aaron: and the whole congregation said unto them, Would God that we had died in the land of Egypt! or would God we had died in this wilderness!

3 And wherefore hath the Lord brought us unto this land, to fall by the sword, that our wives and our children should be a prey? were it not better for us to return into Egypt?

4 And they said one to another, Let us make a captain, and let us return into Egypt.

5 Then Moses and Aaron fell on their faces before all the assembly of the congregation of the children of Israel.

6 And Joshua the son of Nun, and Caleb the son of Jephunneh, which were of them that searched the land, rent their clothes:

7 And they spake unto all the company of the children of Israel, saying, The land, which we passed through to search it, is an exceeding good land.

8 If the Lord delight in us, then he will bring us into this land, and give it us; a land which floweth with milk and honey.

9 Only rebel not ye against the Lord, neither fear ye the people of the land; for they are bread for us: their defence is departed from them, and the Lord is with us: fear them not.

10 But all the congregation bade stone them with stones. And the glory of the Lord appeared in the tabernacle of the congregation before all the children of Israel.

11 And the Lord said unto Moses, How long will this people provoke me? and how long will it be ere they believe me, for all the signs which I have shewed among them?

12 I will smite them with the pestilence, and disinherit them, and will make of thee a greater nation and mightier than they.

13 And Moses said unto the Lord, Then the Egyptians shall hear it, (for thou broughtest up this people in thy might from among them;)

14 And they will tell it to the inhabitants of this land: for they have heard that thou Lord art among this people, that thou Lord art seen face to face, and that thy cloud standeth over them, and that thou goest before them, by day time in a pillar of a cloud, and in a pillar of fire by night.

15 Now if thou shalt kill all this people as one man, then the nations which have heard the fame of thee will speak, saying,

16 Because the Lord was not able to bring this people into the land which he sware unto them, therefore he hath slain them in the wilderness.

17 And now, I beseech thee, let the power of my lord be great, according as thou hast spoken, saying,

18 The Lord is longsuffering, and of great mercy, forgiving iniquity and transgression, and by no means clearing the guilty, visiting the iniquity of the fathers upon the children unto the third and fourth generation.

19 Pardon, I beseech thee, the iniquity of this people according unto the greatness of thy mercy, and as thou hast forgiven this people, from Egypt even until now.

20 And the Lord said, I have pardoned according to thy word:

21 But as truly as I live, all the earth shall be filled with the glory of the Lord.

22 Because all those men which have seen my glory, and my miracles, which I did in Egypt and in the wilderness, and have tempted me now these ten times, and have not hearkened to my voice;

23 Surely they shall not see the land which I sware unto their fathers, neither shall any of them that provoked me see it:

24 But my servant Caleb, because he had another spirit with him, and hath followed me fully, him will I bring into the land whereinto he went; and his seed shall possess it.

25 (Now the Amalekites and the Canaanites dwelt in the valley.) Tomorrow turn you, and get you into the wilderness by the way of the Red sea.

26 And the Lord spake unto Moses and unto Aaron, saying,

27 How long shall I bear with this evil congregation, which murmur against me? I have heard the murmurings of the children of Israel, which they murmur against me.

28 Say unto them, As truly as I live, saith the Lord, as ye have spoken in mine ears, so will I do to you:

29 Your carcases shall fall in this wilderness; and all that were numbered of you, according to your whole number, from twenty years old and upward which have murmured against me.

30 Doubtless ye shall not come into the land, concerning which I sware to make you dwell therein, save Caleb the son of Jephunneh, and Joshua the son of Nun.

31 But your little ones, which ye said should be a prey, them will I bring in, and they shall know the land which ye have despised.

32 But as for you, your carcases, they shall fall in this wilderness.

33 And your children shall wander in the wilderness forty years, and bear your whoredoms, until your carcases be wasted in the wilderness.

34 After the number of the days in which ye searched the land, even forty days, each day for a year, shall ye bear your iniquities, even forty years, and ye shall know my breach of promise.

35 I the Lord have said, I will surely do it unto all this evil congregation, that are gathered together against me: in this wilderness they shall be consumed, and there they shall die.

36 And the men, which Moses sent to search the land, who returned, and made all the congregation to murmur against him, by bringing up a slander upon the land,

37 Even those men that did bring up the evil report upon the land, died by the plague before the Lord.

38 But Joshua the son of Nun, and Caleb the son of Jephunneh, which were of the men that went to search the land, lived still.

39 And Moses told these sayings unto all the children of Israel: and the people mourned greatly.

40 And they rose up early in the morning, and gat them up into the top of the mountain, saying, Lo, we be here, and will go up unto the place which the Lord hath promised: for we have sinned.

41 And Moses said, Wherefore now do ye transgress the commandment of the Lord? but it shall not prosper.

42 Go not up, for the Lord is not among you; that ye be not smitten before your enemies.

43 For the Amalekites and the Canaanites are there before you, and ye shall fall by the sword: because ye are turned away from the Lord, therefore the Lord will not be with you.

44 But they presumed to go up unto the hill top: nevertheless the ark of the covenant of the Lord, and Moses, departed not out of the camp.

45 Then the Amalekites came down, and the Canaanites which dwelt in that hill, and smote them, and discomfited them, even unto Hormah.

I just want to highlight a few verses that indicate that they received according to their belief and words.

Numbers 14:2 And all the children of Israel murmured against Moses and against Aaron: and the whole congregation said unto them, Would God that we had died in the land of Egypt! or would God we had died in this wilderness!

Numbers 14:28 Say unto them, As truly as I live, saith the Lord, as ye have spoken in mine ears, so will I do to you:

29 Your carcases shall fall in this wilderness; and all that were numbered of you, according to your whole number, from twenty years old and upward which have murmured against me.

Scripture is consistent within itself to point out ongoing themes that are important.  The importance and connection between faith and words is in multiple places in scriptures.  The only way to cover them all is to get a good concordance and look them up.   A few well-known key verses are listed here.

Proverbs 18:21 Death and life are in the power of the tongue: and they that love it shall eat the fruit thereof.

 

James 1:5 If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him.

 

James 1:6 But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed.

7 For let not that man think that he shall receive any thing of the Lord.

8 A double minded man is unstable in all his ways.

 

Matthew 12:33 Either make the tree good, and his fruit good; or else make the tree corrupt, and his fruit corrupt: for the tree is known by his fruit.

34 O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh.

35 A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.

36 But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment.

37 For by thy words thou shalt be justified, and by thy words thou shalt be condemned.