This claim is a clear bent towards Calvinism (if not even hyper-calvinism/hyper-sovereignty). We cannot pull isolated scriptures to support a doctrine. We must look at the whole Bible context, especially when dealing with Proverbs, which are general principles stated without a context. Sure God can turn a man's heart. However, to say that God turns all hearts, whether evil or good, is a clear violation of James 1:13, not to mention a very scary heretical notion -- woe to them who call evil good (Isa 5:20).
We must realize that "the king's heart" in Proverbs 21:1 is referring to one king (it's singular). Solomon was likely speaking of his own heart being turned by God --- As he submitted to God. A simple observation of the 32 occurrences of the Hebrew word melek ("king") in the book of Proverbs proves my assertion:
(Pro 1:1) The proverbs of Solomon the son of David, king of Israel;
(Pro 8:15) By me kings reign, and princes decree justice.
(Pro 14:28) In the multitude of people is the king's honour: but in the want of people is the destruction of the prince.
(Pro 14:35) The king's favour is toward a wise servant: but his wrath is against him that causeth shame.
(Pro 16:10) A divine sentence is in the lips of the king: his mouth transgresseth not in judgment.
(Pro 16:12) It is an abomination to kings to commit wickedness: for the throne is established by righteousness.
(Pro 16:13) Righteous lips are the delight of kings; and they love him that speaketh right.
(Pro 16:14) The wrath of a king is as messengers of death: but a wise man will pacify it.
(Pro 16:15) In the light of the king's countenance is life; and his favour is as a cloud of the latter rain.
(Pro 19:12) The king's wrath is as the roaring of a lion; but his favour is as dew upon the grass.
(Pro 20:2) The fear of a king is as the roaring of a lion: whoso provoketh him to anger sinneth against his own soul.
(Pro 20:8) A king that sitteth in the throne of judgment scattereth away all evil with his eyes.
(Pro 20:26) A wise king scattereth the wicked, and bringeth the wheel over them.
(Pro 20:28) Mercy and truth preserve the king: and his throne is upholden by mercy.
(Pro 21:1) The king's heart is in the hand of the LORD, as the rivers of water: he turneth it whithersoever he will.
(Pro 22:11) He that loveth pureness of heart, for the grace of his lips the king shall be his friend.
(Pro 22:29) Seest thou a man diligent in his business? he shall stand before kings; he shall not stand before mean men.
(Pro 24:21) My son, fear thou the LORD and the king: and meddle not with them that are given to change:
(Pro 25:1) These are also proverbs of Solomon, which the men of Hezekiah king of Judah copied out.
(Pro 25:2) It is the glory of God to conceal a thing: but the honour of kings is to search out a matter.
(Pro 25:3) The heaven for height, and the earth for depth, and the heart of kings is unsearchable.
(Pro 25:5) Take away the wicked from before the king, and his throne shall be established in righteousness.
(Pro 25:6) Put not forth thyself in the presence of the king, and stand not in the place of great men:
(Pro 29:4) The king by judgment establisheth the land: but he that receiveth gifts overthroweth it.
(Pro 29:14) The king that faithfully judgeth the poor, his throne shall be established for ever.
(Pro 30:27) The locusts have no king, yet go they forth all of them by bands;
(Pro 30:28) The spider taketh hold with her hands, and is in kings' palaces.
(Pro 30:31) A greyhound; an he goat also; and a king, against whom there is no rising up.
(Pro 31:1) The words of king Lemuel, the prophecy that his mother taught him.
(Pro 31:3) Give not thy strength unto women, nor thy ways to that which destroyeth kings.
(Pro 31:4) It is not for kings, O Lemuel, it is not for kings to drink wine; nor for princes strong drink:
From this list of Proverbs, we see the following characteristics of the king:
- The kings and princes decree "justice" by wisdom
- The king shows favor towards a wise servant
- The kings mouth transgresses not in judgment
- The king delights in righteous lips and love them that speak right
- The king's throne is established in righteousness
- The king scatters away evil and the wicked (Wouldn't apply to an evil king -- 2 Chr 12:13-16)
- The kings is preseved by mercy and truth
In 29:4 we see a clear distinction between the "king" (i.e. the righteous king) and "he that receiveth gifts" (i.e. a greedy king who forces unnecessary taxation). So it is quite evident that a majority of general verses about the "king" are implying a righteous king (and perhaps Solomon himself). Since there were various kings who didn't fit the above characteristics in the history of the northern and southern kingdoms, we must assume that most of these verses are not including them.
People often use Prov 16:9 in a similar manner, because they lack the diligence to do word studies such as they should:
Pr 16:9 A man’s heart deviseth his way: but the LORD directeth {Hebrew Kuwn} his steps.
Based on this verse they think God is the uncaused cause of every action of every individual (including sin). However, they fail to see the same Hebrew verb used 6 verses earlier which shows that this "directing, establishing, turning" is conditional of "commiting thy works unto the Lord". This shows a synergism between God and man.
Pr 16:3 Commit thy works unto the LORD, and thy thoughts shall be established (Hebrew Kuwn)
In the Proverb originally referenced (Prov 21), it clearly states that God "directs" the upright, not the wicked:
Pr 21:29 A wicked man hardeneth his face: but as for the upright, he directeth {Hebrew kuwn} his way
In addition, God will direct a humble believer who seeks His will (implying that those that are not humble will not be directed):
Psa 10:17 LORD, thou hast heard the desire of the humble: thou wilt prepare {Hebrew kuwn} their heart, thou wilt cause thine ear to hear:
God will direct the righteous man in whom He is delighted (imlying that the steps of the wicked are not directed):
Psa 37:23 The steps of a good man are ordered {Hebrew kuwn} by the LORD: and he delighteth in his way.
Israel was not directed by God in the wilderness:
Psa 78:37 For their heart was not right {Hebrew kuwn} with him, neither were they stedfast in his covenant.
Therefore, to be directed by God requires your trust and acknowledgment of His ways:
Prov 3:5 Trust in the LORD with all thine heart; and lean not unto thine own understanding. 6 In all thy ways acknowledge him, and he shall direct thy paths.
Rom 8:14 For as many as are led by the Spirit of God, they are the sons of God.
CAN GOD TURN ANY HEART? YES. DOES GOD TURN EVERY HEART? NO