Objection 1. It would seem that it is not a sin to tempt God. For God has not commanded sin. Yet He has commanded men to try, which is the same as to tempt, Him: for it is written (Malachi 3:10): "Bring all the tithes into the storehouse, that there may be meat in My house; and try Me in this, saith the Lord, if I open not unto you the flood-gates of heaven." Therefore it seems not to be a sin to tempt God.
Objection 2. Further, a man is tempted not only in order to test his knowledge and his power, but also to try his goodness or his will. Now it is lawful to test the divine goodness or will, for it is written (Psalm 33:9): "O taste and see that the Lord is sweet," and (Romans 12:2): "That you may prove what is the good, and the acceptable, and the perfect will of God." Therefore it is not a sin to tempt God.
Objection 3. Further, Scripture never blames a man for ceasing from sin, but rather for committing a sin. Now Achaz is blamed because when the Lord said: "Ask thee a sign of the Lord thy God," he replied: "I will not ask, and I will not tempt the Lord," and then it was said to him: "Is it a small thing for you to be grievous to men, that you are grievous to my God also?" (Isaiah 7:11-13). And we read of Abraham (Genesis 15:8) that he said to the Lord: "Whereby may I know that I shall possess it?" namely, the land which God had promised him. Again Gedeon asked God for a sign of the victory promised to him (Judges 6:36, sqq.). Yet they were not blamed for so doing. Therefore it is not a sin to tempt God.
On the contrary, It is forbidden in God's Law, for it is written (Deuteronomy 6:10): "Thou shalt not tempt the Lord thy God."
I answer that, As stated above (Article 1), to tempt a person is to put him to a test. Now one never tests that of which one is certain. Wherefore all temptation proceeds from some ignorance or doubt, either in the tempter (as when one tests a thing in order to know its qualities), or in others (as when one tests a thing in order to prove it to others), and in this latter way God is said to tempt us. Now it is a sin to be ignorant of or to doubt that which pertains to God's perfection. Wherefore it is evident that it is a sin to tempt God in order that the tempter himself may know God's power.
On the other hand, if one were to test that which pertains to the divine perfection, not in order to know it oneself, but to prove it to others: this is not tempting God, provided there be just motive of urgency, or a pious motive of usefulness, and other requisite conditions. For thus did the apostles ask the Lord that signs might be wrought in the name of Jesus Christ, as related in Acts 4:30, in order, to wit, that Christ's power might be made manifest to unbelievers.
Reply to Objection 1. The paying of tithes was prescribed in the Law, as stated above (Question 87, Article 1). Hence there was a motive of urgency to pay it, through the obligation of the Law, and also a motive of usefulness, as stated in the text quoted--"that there may be meat in God's house": wherefore they did not tempt God by paying tithes. The words that follow, "and try Me," are not to be understood causally, as though they had to pay tithes in order to try if "God would open the flood-gates of heaven," but consecutively, because, to wit, if they paid tithes, they would prove by experience the favors which God would shower upon them.
Reply to Objection 2. There is a twofold knowledge of God's goodness or will. One is speculative and as to this it is not lawful to doubt or to prove whether God's will be good, or whether God is sweet. The other knowledge of God's will or goodness is effective or experimental and thereby a man experiences in himself the taste of God's sweetness, and complacency in God's will, as Dionysius says of Hierotheos (Div. Nom. ii) that "he learnt divine thing through experience of them." It is in this way that we are told to prove God's will, and to taste His sweetness.
Reply to Objection 3. God wished to give a sign to Achaz, not for him alone, but for the instruction of the whole people. Hence he was reproved because, by refusing to ask a sign, he was an obstacle to the common welfare. Nor would he have tempted God by asking, both because he would have asked through God commanding him to do so, and because it was a matter relating to the common good. Abraham asked for a sign through the divine instinct, and so he did not sin. Gedeon seems to have asked a sign through weakness of faith, wherefore he is not to be excused from sin, as a gloss observes: just as Zachary sinned in saying to the angel (Luke 1:18): "Whereby shall I know this?" so that he was punished for his unbelief.
It must be observed, however, that there are two ways of asking God for a sign: first in order to test God's power or the truth of His word, and this of its very nature pertains to the temptation of God. Secondly, in order to be instructed as to what is God's pleasure in some particular matter; and this nowise comes under the head of temptation of God.
Source: Summa Theologica (New Advent)