"You shall not put the LORD your God to the test, as you tested Him at Massah. (Deuteronomy 6:16)
The literal commandment is not to put the LORD your G-d to the test.
After fasting in the wilderness for forty days, Messiah Yeshua is tempted by the devil. One of the temptations was for Yeshua to cast Himself down from the pinnacle of the Temple. "After all", the devil says, "'He will command His angels concerning You' and 'on their hands they will bear You up, so that You will not strike your foot against a stone.'"
The Master resists this temptation and responds by quoting this commandment:
You shall not put the LORD your G-d to the test.
The devil was tempting Yeshua to sin by asking G-d to prove Himself. Yeshua knew this was a sin and resisted.
Messiah exemplified this commandment in several ways (one being the literal example above). Another way that this verse pictures Messiah is through the intimate relationship He has with the Father.
In John 10:30 Yeshua declares "I and the Father are one." The Son knows the Father so well that there is no need to put G-d to the test and ask Him to prove Himself. The need for such proof comes as a result of distance and a lack of a close relationship. Because Yeshua and the Father are one, there is zero distance between them and zero need for G-d to prove Himself.
In a similar way Messiah calls us to be in Him as He is in the Father (John 17:21). We are called to have an intimate relationship with G-d. As a result of such a relationship we should know G-d is Who He claims to be and should not test Him. Such a test is both a sin and evidence of a lack of faith and a proper relationship to G-d.
Messiah fulfilled this commandment by not testing G-d. Even when He had been fasting for forty days and was physically weak and tempted by the devil himself, Yeshua resisted and did not test G-d.
This commandment is traditionally understood as "do not test the prophet unduly" and observance of this negative command involves not asking G-d to prove Himself by giving you something or doing some miracle.
Humble, prayerful requests for genuine needs for yourself or others are appropriate but should not be used to get G-d to prove Who He says he is.
We find this commandment affirmed in Matthew 4 during Messiah's testing in the wilderness:
Then the devil took Him into the holy city and had Him stand on the pinnacle of the temple, and said to Him, "If You are the Son of God, throw Yourself down; for it is written, 'HE WILL COMMAND HIS ANGELS CONCERNING YOU'; and 'ON their HANDS THEY WILL BEAR YOU UP, SO THAT YOU WILL NOT STRIKE YOUR FOOT AGAINST A STONE.'"Jesus said to him, "On the other hand, it is written, 'YOU SHALL NOT PUT THE LORD YOUR GOD TO THE TEST.'" (Matthew 4:5-7)
This is also recorded in Luke 4:12.
It is interesting to note that the same Greek for "test" (ἐκπειράζω ekpeirazō) is found in Luke 10:
And a lawyer stood up and put Him to the test, saying, "Teacher, what shall I do to inherit eternal life?" (Luke 10:25)
This lawyer was putting Yeshua to the test!
Paul exhorts those in Corinth to avoid this sin:
Now these things happened as examples for us, so that we would not crave evil things as they also craved. Do not be idolaters, as some of them were; as it is written, "THE PEOPLE SAT DOWN TO EAT AND DRINK, AND STOOD UP TO PLAY." Nor let us act immorally, as asome of them did, and twenty-three thousand fell in one day. Nor let us try [ekpeirazō] the Lord, as some of them did, and were destroyed by the serpents. Nor grumble, as some of them did, and were destroyed by the destroyer. (1 Corinthians 10:6-10)
We are able to fulfill this commandment today and we should not test G-d unduly.
What does that mean?
This commandment provides a specific example of what not to do: test G-d as the Israelites tested G-d at Massah. So... what happened at Massah?
Then all the congregation of the sons of Israel journeyed by stages from the wilderness of Sin, according to the command of the LORD, and camped at Rephidim, and there was no water for the people to drink. Therefore the people quarreled with Moses and said, "Give us water that we may drink." And Moses said to them, "Why do you quarrel with me? Why do you test the LORD?" But the people thirsted there for water; and they grumbled against Moses and said, "Why, now, have you brought us up from Egypt, to kill us and our children and our livestock with thirst?" So Moses cried out to the LORD, saying, "What shall I do to this people? A little more and they will stone me." (Exodus 17:1-4)
He named the place Massah [testing] and Meribah [quarreling] because of the quarrel of the sons of Israel, and because they tested the LORD, saying, "Is the LORD among us, or not?" (Exodus 17:7)
The specific test given here as an example is asking G-d to prove Himself. This is similar to a petulant child saying "If you love me you will do such and such a thing".
The literal commandment says we should not test G-d in this manner.
Although Gideon "put G-d to the test" using fleece (see Judges 6:36-40) it was with a sincere "I just want to make sure I am doing the right thing" attitude that was completely different from what was seen at Massah.
In his letter to the believers in Philippi, Paul writes:
Be anxious for nothing, but in everything by prayer and supplication with thanksgiving let your requests be made known to God. And the peace of God, which surpasses all comprehension, will guard your hearts and your minds in the Messiah Yeshua. (Philippians 4:6-7)
We absolutely should pray with faith that G-d is able to do all things.