In English, the word "doubt" means to be uncertain or unsure about something, skeptical about, consider questionable or unlikely, distrust, not be sure, to lack confidence in.
In English, the word "doubt" would be another word for lack of faith.
Hebrews 11:1 defines faith: "Faith is being sure of what we hope for, certain of what we do not see." (NIV'78)
We find the word "doubt" in English translations of the New Testament, but the English word does not have exactly the same corresponding meaning as the Greek words from which it is translated.
In the New Testament, there are two main words translated "doubt" in connection with faith, "dia-krino" and "di-stasis," and maybe a third, "mete-orizo."
"Di-stasis" literally means to "two-stand." In Matt 14:31, Peter was walking on the water, considered the strong wind, became afraid, sank, and Jesus literally said to him, "...why do you two-stand [Strong's G1365, inflected εδιστασας, "e-di-stasas"]?"
The only other place this word occurs is in Matt 28:17, where the 11 disciples "worshiped" him, and literally "yet they-two-stand [εδιστασαν]."
So that's an easy word to understand, at face value. In issues of faith, we need to take "one stand" and stick with it, not "two stands."
"Mete-orizo" only occurs once in the New Testament, in Luke 12:29, where Jesus says "and do not be seeking what you may eat or what you may drink, and be not being in suspense [Strong's G3349, inflected μετεωριζεσθε, "mete-orezesthe"]." The "meta" part means "change, transition, in-between", "orizo" means "define/specify/bound," so "mete-orizo" means to be "in transition/in-between/change of decision."
So, obviously, we should not be wavering in decision, being unsettled in our minds about where our next meal is coming from, but have faith that God will provide these things. That's also an easy word to understand.
The word much more commonly translated "doubt" is διακρινω (Strong's G1252, "dia-krino"), which literally says "through-judge." That word requires a little more consideration to grasp the concept.
The "krino" part of the word (Strong's G2919, κρινω), alone, would just mean to "judge," that is, to decide a matter. The "dia" = "through" part of the word (Strong's G1223, δια) implies a process by which you would make that decision.
So "dia-krino" means to scrutinize, examine, litigate, discern, discriminate, deliberate through, etc.
Now, this "dia-krino" can be a good thing. And in the Bible, many times it is used in a positive sense, whereas the English word "doubt" is always negative. For example, in 1 Cor 11:29, we are supposed to "discern" ("dia-krino") the body of the Lord when partaking of the Lord's Supper. In 1 Cor 6:5, they ought to have had someone in the church who was wise enough to adjudicate ("dia-krino") disputes, rather than suing each other in secular courts of law. In 1 Cor 14:29, "two or three prophets should speak," and others should "discriminate" ("dia-krino") what is said. In Heb 5:14, we should learn to "discriminate" ("dia-krino") between good and evil.
The issue is that when it comes to "just believing" the Word of God and his promises, we should not "dia-krino." We should not discriminate between the Word of God or his promises vs. anything else. We should not consider any other options. There is nothing to deliberate about, nothing to scrutinize, nothing to bring into question. God says it or promises it, and that settles it. It's not the Word of God vs. something else. It's not we "believe" and then question or deliberate about whether that or something else is true.
So to make a true stand in faith means that you have settled the matter and will consider no other alternatives. Circumstances that your five physical senses tell you don't matter. Dis-information from the devil or other people or thoughts from out of your carnal, unrenewed mind are not to be taken into account. The matter is already settled and decided, so there is nothing to scrutinize or deliberate about, nothing to discern. You "just believe." You "only believe."
In Matt 21:21, Mark 11:23, and James 1:6 it says "believe and do not doubt ('dia-krino')." In Romans 4:20 it says that Abraham "did not doubt ('dia-krino') in unbelief."
"...he must believe and not doubt ('dia-krino'), for he who doubts ('dia-krino') is like a wave of the sea, blown and tossed by the wind. That man should not think he will receive anything from the Lord..." (James 1:6-7)
It is interesting that James goes on to say that such a man is "double-life-ed," turbulent in all his ways" (James 1:8). That is "di-psuchos," meaning that such a person's "life" goes two ways.
To help get a sense of the usage of "dia-krino" I have cited not only all the occurrences of the Greek word in the New Testament, but also all the occurrences of the Greek word in the Septuagint, which is the Greek translation of the Old Testament that was in use at the time of Jesus.
Here they all are (English OT translation from the ABP, NT from CLV):
Exo 18:16 (Moses to Jethro) "for whenever there is a dispute, and they should come to me, I litigate each...""Di-stasis" does not occur anywhere in the Septuagint.
Lev 24:12 "And they put him [the man who cursed the name of the Lord] in prison, to litigate over him because of the order of the Lord."
Deut 33:7 "...His hands shall litigate for him..."
2 Sam 19:9 "And all the people were arguing among all the tribes of Israel, saying..."
1 Kings 3:9 "And you shall give to your servant a heart to hear and to litigate your people in righteousness" (Solomon's plea for wisdom)
1 Chron 26:29 "...to act as scribe and to litigate..."
Job 9:14 (Job to Bildad) "But then should he obey me, or shall he litigate my things?
Job 12:11 (Job to friends) "For the ear indeed scrutinizes words..."
Job 15:5 (Eliphaz to Job) "You are liable by the sayings of your mouth, of which you litigated sayings of mighty ones."
Job 21:22 (Job to friends) "Is it not the Lord who is the one teaching understanding and higher knowledge? And is it not he who litigates the wise?"
Job 23:10 (Job to friends) "For he knows my way already; he examined me as the gold."
Job 37:16 (Elihu to Job) "And he knows the distinction of clouds, and the extraordinary calamitous downfalls of the wicked."
Psalm 50:4-5 "He shall call on the heaven upward, and the earth, to separate his people. Gathered to him are his sacred ones, the ones ordaining his covenant..."
Psalm 82:1 "God stood in the congregation of gods; and in the midst of gods he examines. For how long will you judge injustice ["judge" = "krino"].
Prov 31:9 "Open your mouth, and judge ["judge" = "krino"] justly, and litigate for the needy and weak!"
Eccl 3:18 "I said in my heart, concerning the speech of the sons of man, that God will examine them, and to show that they are beasts."
Jer 15:10 "Woe, O mother, as how you bore me a man adjudicating [= "dikaio" = justifying] and litigating in all the earth.
Ezek 17:20 "...And I will lead him into Babylon. And I shall litigate with him there concerning his iniquity of which he transgressed against me."
Ezek 20:35 "And I will lead you into the wilderness of the peoples, and I will litigate for you there face to face."
Ezek 20:36 "In which manner I litigated for your fathers in the wilderness of the land of Egypt, so I will judge you ["judge" = "krino"]"
Ezek 34:17 "...Behold, I separate between sheep and sheep, rams and he-goats."
Ezek 34:20 "...Behold, I separate between the strong sheep and weak."
Ezek 44:24 "And these shall attend over a judgment ["krino"] of blood, to litigate my ordinances; they shall do justice ["dikaio"]..."
Joel 3:2 "...I will lead them into the valley of Jehoshaphat. And I will litigate with them there for my people..."
Joel 3:12 "...for there I will sit to separate all the nations round about."
Zechariah 3:7 "...then you shall litigate my house..."
Matt 16:3 "...you hypocrites! You can discern the face of the sky, but not the signs of the times?"
Matt 21:21 "Now, answering, Jesus said to them, 'Verily I am saying to you, if you should be having faith and not be doubting, not only that to the fig tree will you be doing, but if you should be saying to this mountain also, "Be picked up and cast into the sea!" it will be occuring'"
Mark 11:23 "Verily, I am saying to you that whosoever may be saying to this mountain, 'Be picked up and cast into the sea,' and may not be doubting in his heart, but should be believing that what he is speaking is occurring, it shall be his, whatsoever he may be saying."
Acts 10:20 "But, rising, descent and go with them, nothing doubting, for I have commissioned them."
Acts 11:2 "Now when Peter went up into Jerusalem, those of the circumcision doubted him, saying that 'You entered to men having uncircumcision, and you ate with them!'"
Acts 11:12 "Now the Spirit said to me to come together with them, nothing doubting. Now these six brethren..."
Acts 15:9 "And in nothing discriminates between us and them, cleansing their hearts by faith."
Rom 4:20 "yet the promise of God was not doubted in unbelief, but he was invigorated by faith, giving glory to God."
Rom 14:1 "Now the infirm in the faith be taking to yourselves, but not for discrimination of reasonings."
Rom 14:23 "Now he who is doubting, if he should be eating is condemned, seeing that it is not out of faith. Now everything which is not out of faith is sin."
1 Cor 4:7 "For who is making you to discriminate?
1 Cor 6:5 "To abash you am I saying this. Thus is there not among you one wise man who will be able to adjudicate amidst his brethren?
1 Cor 11:29 "For he who is eating and drinking unworthily is eating and drinking judgment ["krino"] to himself, not discriminating the body of the Lord."
1 Cor 11:31 "For if we adjudicated ourselves, we would not be judged ["krino"]"
1 Cor 12:10 "...yet to another discrimination of spirits..."
1 Cor 14:29 "Now, as to prophets, let two or three speak, and let the others discriminate"
Heb 5:14 "Now solid nourishment is for the mature, who, because of habit, have faculties exercised for discriminating between the ideal and the evil."
James 1:6 "Yet let him be requesting in faith, doubting nothing, for he who is "doubting simulates a surge of the sea, driven by the wind and tossed."
James 2:4 "were you not discriminating among yourselves, and did you not become judges with wicked reasonings?"
Jude 1:9 "Now when Michael, the chief messenger, doubting the Adversary, argued concerning the body of Moses..."
Jude 1:22 "And to those, indeed, who are doubting, be merciful."
"Mete-orizo" occurs in the Septuagint in the following places: Jdg 1:15, Ps 42:7, 88:7, 93:4, 131:1, Job 28:18, Isa 2:12,13, 5:15, 17:6, 18:2, 30:25, 52:13, 57:7 Jer 31:37, 32:17, Eze 3:14,15, 10:16,17,19, Jon 2:3, Ob 1:4, Mic 4:1.
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