Did he do more than slay Goliath?
David is important because God anointed him king over Israel,
Israel was the kingdom of God at the time because Solomon the son of David is said to have ‘sat on the throne of the LORD’.
1 Chronicles 29:23 Then Solomon sat on the throne of the LORD as king instead of David his father, and prospered; and all Israel obeyed him.
Jeremiah prophesies that this throne of the Lord will be restored.
Jeremiah 3:17 At that time they shall call Jerusalem the throne of the LORD; and all the nations shall be gathered unto it, to the name of the LORD, to Jerusalem: neither shall they walk any more after the imagination of their evil heart.
David is important because God promised him a descendant who would establish his kingdom and reign forever.
He also promised that God would be his father.
2 Samuel 7:12 And when thy days be fulfilled, and thou shalt sleep with thy fathers, I will set up thy seed after thee, which shall proceed out of thy bowels, and I will establish his kingdom. 13 He shall build an house for my name, and I will stablish the throne of his kingdom for ever. 14 I will be his father, and he shall be my son.
Gabriel said of Jesus:
Luke 1:32 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: 33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
When king Zedekiah was taken to Babylon God said that the kingdom would be overturned and there would be no more kings over Israel until he came whose right it is.
Ezekiel 21:27 I will overturn, overturn, overturn, it: and it shall be no more, until he come whose right it is; and I will give it him.
Jesus is the promised son of David.
Matthew 1:1 The book of the generation of Jesus Christ, the son of David, the son of Abraham.
He preached the gospel of the kingdom of God.
Mark 1:14 Now after that John was put in prison, Jesus came into Galilee, preaching the gospel of the kingdom of God,
The kingdom of God is the restored kingdom of David.
The disciples asked:
Acts 1:6… Lord, wilt thou at this time restore again the kingdom to Israel?
The restoration is still in the future.
When Christ returns he will establish the kingdom of God and rule over the earth from Jerusalem.
Israel will be restored to their land as described in Ezekiel 48.
Isaiah speaks of this time:
Isaiah 2:2 And it shall come to pass in the last days, that the mountain of the LORD’S house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it. 3 And many people shall go and say, Come ye, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem. 4 And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruninghooks: nation shall not lift up sword against nation, neither shall they learn war any more.
In this kingdom the apostles, raised from the dead to eternal life, will be appointed judges over the tribes of Israel.
Matthew 19:28… Jesus said unto them, Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.
Psalm 2 speaks of this time
Psalm 2:1 Why do the heathen rage, and the people imagine a vain thing? 2 The kings of the earth set themselves, and the rulers take counsel together, against the LORD, and against his anointed, saying, 3 Let us break their bands asunder, and cast away their cords from us. 4 He that sitteth in the heavens shall laugh: the Lord shall have them in derision. 5 Then shall he speak unto them in his wrath, and vex them in his sore displeasure. 6 Yet have I set my king upon my holy hill of Zion. 7 I will declare the decree: the LORD hath said unto me, Thou art my Son; this day have I begotten thee. 8 Ask of me, and I shall give thee the heathen for thine inheritance, and the uttermost parts of the earth for thy possession. 9 Thou shalt break them with a rod of iron; thou shalt dash them in pieces like a potter’s vessel. 10 Be wise now therefore, O ye kings: be instructed, ye judges of the earth. 11 Serve the LORD with fear, and rejoice with trembling. 12 Kiss the Son, lest he be angry, and ye perish from the way, when his wrath is kindled but a little. Blessed are all they that put their trust in him.
This initial rule over the mortal population of the earth will last 1000 years.
At the end of this time there will be a second resurrection and final judgment when sin and death will be eradicated and God will be all in all.
Paul describes what is going to happen:
1 Corinthians 15:20 But now is Christ risen from the dead, and become the firstfruits of them that slept. 21 For since by man came death, by man came also the resurrection of the dead. 22 For as in Adam all die, even so in Christ shall all be made alive. 23 But every man in his own order: Christ the firstfruits; afterward they that are Christ’s at his coming. 24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power. 25 For he must reign, till he hath put all enemies under his feet. 26 The last enemy that shall be destroyed is death. 27 For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him. 28 And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.
I hope you find this helpful.