The noun ‘doubt’ means:

A feeling of uncertainty or lack of conviction.

“some doubt has been cast upon the authenticity of this account”

synonyms: uncertainty, unsureness, indecision, hesitation, dubiousness, suspicion, confusion

antonyms: certainty, confidence, conviction, trust

Two world-historical persons faced life threatening doubt in their lifetimes. One chose poorly which resulted in death for himself, his wife, and all his children and the Other chose well which resulted in everlasting life for those who are His. These are their stories.

The first story starts and ends in a garden:

Now the serpent was more crafty than any other beast of the field that the Lord God had made.

He said to the woman, “Did God actually say, ‘You shall not eat of any tree in the garden’?” And the woman said to the serpent, “We may eat of the fruit of the trees in the garden, but God said, ‘You shall not eat of the fruit of the tree that is in the midst of the garden, neither shall you touch it, lest you die.’”

Genesis 3:1-3 English Standard Version (ESV)

To this, the commentator, John Calvin, said:

Yea, has God said? …More correct is the [translation…] ‘Can it be, that God should forbid the eating of any tree whatever?’ …because there is greater probability that Satan, in order to deceive more covertly, would gradually proceed with cautious prevarications to lead the woman to a contempt of the divine precept.

…Under the pretext of inquiring into the cause, [Satan] indirectly weakens [Adam’s and Eve’s] confidence in [God’s] word. …I have no doubt that the serpent urges the woman to seek out the cause, since otherwise he would not have been able to draw away her mind from God.

Very dangerous is the temptation, when it is suggested to us, that God is not to be obeyed except so far as the reason of his command is apparent. The true rule of obedience is, that we, being content with a bare command, should persuade ourselves that whatever he enjoins is just and right.

But whosoever desires to be wise beyond measure, him will Satan, seeing he has cast off all reverence for God, immediately precipitate into open rebellion…Satan…wished to inject into the woman a doubt which might induce her to believe [what God had said] not to be the word of God…

The second story starts in a wilderness and ends on a site of execution:

[Jesus…was led by the Spirit in the wilderness] for forty days, being tempted by the devil. And he ate nothing during those days. And when they were ended, he was hungry. The devil said to him, “If you are the Son of God, command this stone to become bread.” And Jesus answered him, “It is written, ‘Man shall not live by bread alone.’” Luke 4:2-4 (ESV)

and

After fasting forty days and forty nights, he was hungry. And the tempter came and said to him, “If you are the Son of God, command these stones to become loaves of bread.” But he answered, “It is written,

“‘Man shall not live by bread alone,

    but by every word that comes from the mouth of God.’”

Matthew 4:2-4 (ESV)

To this temptation, Calvin said:

Christ’s reply…is appropriate: “Man shall not live by bread alone.” […It is as if He said:] “You advise me to contrive some remedy, for obtaining relief in a different manner from what God permits. This would be to distrust God; and I have no reason to expect that he will support me in a different manner from what he has promised in his word. You, Satan, represent his favor as confined to bread: but Himself declares, that, though every kind of food were wanting, his blessing alone is sufficient for our nourishment.”

Such was the kind of temptation which Satan employed, the same kind with which he assails us daily. The Son of God did not choose to undertake any contest of an unusual description, but to sustain assaults in common with us, that we might be furnished with the same armor, and might entertain no doubt as to achieving the victory…

Next, he unravels the often-unacknowledged truth of our daily sustenance:

The word does not mean doctrine, but the purpose which God has made known, with regard to preserving the order of nature and the lives of his creatures. Having created men, he does not cease to care for them: but, as “he breathed into their nostrils the breath of life,” (Genesis 2:7,) so he constantly preserves the life which he has bestowed.

In like manner, the Apostle says, that he “upholds all things by his powerful word,” (Hebrews 1:3) that is, the whole world is preserved, and every part of it keeps its place, by the will and decree of Him, whose power, above and below, is everywhere diffused. Though we live on bread, we must not ascribe the support of life to the power of bread, but to the secret kindness, by which God imparts to bread the quality of nourishing our bodies.

Hence, also, follows another statement: by every word that proceeds out of the mouth of God shall men live. God, who now employs bread for our support, will enable us, whenever he pleases, to live by other means…

Finally, Calvin sums up this lesson as:

The precise object of Christ’s reply is this: We ought to trust in God for food, and for the other necessaries of the present life, in such a manner, that none of us may overleap the boundaries which he has prescribed.

But if Christ did not consider himself to be at liberty to change stones into bread, without the command of God, much less is it lawful for us to procure food by fraud, or robbery, or violence, or murder.

And this second story ends outside Jerusalem:

Those who passed by derided him, wagging their heads and saying, “You who would destroy the temple and rebuild it in three days, save yourself! If you are the Son of God, come down from the cross.” Matthew 27:39-40 (ESV)

About these verses, Calvin discusses the question:

If thou art the Son of God. Wicked men demand from Christ such a proof of His power that, by proving himself to be the Son of God, he may cease to be the Son of God. He had clothed himself with human flesh, and had descended into the world, on this condition, that, by the sacrifice of his death, he might reconcile men to God the Father.

So then, in order to prove himself to be the Son of God, it was necessary that he should hang on the cross. [Yet,] those wicked men [argued] that the Redeemer will not be recognized as the Son of God, unless he come down from the cross, [disobeying] the command of his Father, and, leaving incomplete the expiation of sins, [thus] divesting himself of the office which God had assigned to him.

But let us learn from [their evil witness] to confirm our faith by considering that the Son of God determined to remain nailed to the cross for the sake of our salvation, until he had endured most cruel torments of the flesh, and dreadful anguish of soul, and even death itself.

And lest we should come to tempt God in a manner similar to that in which those men tempted him, let us allow God to conceal his power, whenever it pleases Him to do so, that he may afterwards display it at his pleasure at the proper time and place.

And so God deigned to show us favor by resisting the temptation and triumphing over death.

On the same topic, the preacher, Charles Spurgeon, offers solace and encouragement in the face of many such ‘ifs’ that cast doubt.

Therefore, no longer doubt, but believe in Him.

R.C. Sproul: Christ Crucified, YouTube, Ligonier Ministries