Even before David fought Goliath, God had chosen David to replace Saul as king and David was anointed by the prophet Samuel as king, replacing Saul.
I Samuel 16:13 Then Samuel took the horn of oil, and anointed him in the midst of his brethren: and the Spirit of the LORD came upon David from that day forward. So Samuel rose up, and went to Ramah.
The Holy Spirit had been removed from Saul and the Holy Spirit was given to David, and Samuel the prophet of God had anointed David by the Word of God to be king over Israel.
Though God had made these changes, they didn't come to pass in the eyes of the people until later. Saul remained king and David was little known. But in the spiritual kingdom David replaced Saul as king and it would be seen by all in time.
In the meantime, Saul keeps David with him and even makes David to be in charge of his army.
(But David is already anointed by God as king to replace Saul. I Samuel 16)
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I Samuel 18
6 And it came to pass as they came, when David was returned from the slaughter of the Philistine, that the women came out of all cities of Israel, singing and dancing, to meet king Saul, with tabrets, with joy, and with instruments of musick.
7 And the women answered one another as they played, and said, Saul hath slain his thousands, and David his ten thousands.
8 And Saul was very wroth, and the saying displeased him; and he said, They have ascribed unto David ten thousands, and to me they have ascribed but thousands: and what can he have more but the kingdom?
9 And Saul eyed David from that day and forward.
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And God has removed the Holy Spirit from Saul, and God has given HIS Holy Spirit to David.
So David has the Holy Spirit and Saul has been given, "an evil spirit from the Lord."
I Samuel 16
14 But the Spirit of the LORD departed from Saul, and an evil spirit from the LORD troubled him.
14 But the Spirit of the LORD departed from Saul, and an evil spirit from the LORD troubled him.
16 Let our lord now command thy servants, which are before thee, to seek out a man, who is a cunning player on an harp: and it shall come to pass, when the evil spirit from God is upon thee, that he shall play with his hand, and thou shalt be well.
17 And Saul said unto his servants, Provide me now a man that can play well, and bring him to me.
18 Then answered one of the servants, and said, Behold, I have seen a son of Jesse the Bethlehemite, that is cunning in playing, and a mighty valiant man, and a man of war, and prudent in matters, and a comely person, and the LORD is with him.
19 Wherefore Saul sent messengers unto Jesse, and said, Send me David thy son, which is with the sheep.
20 And Jesse took an ass laden with bread, and a bottle of wine, and a kid, and sent them by David his son unto Saul.
21 And David came to Saul, and stood before him: and he (David) loved him (Saul) greatly; and he became his armourbearer.
22 And Saul sent to Jesse, saying, Let David, I pray thee, stand before me; for he hath found favour in my sight.
23 And it came to pass, when the evil spirit from God was upon Saul, that David took an harp, and played with his hand: so Saul was refreshed, and was well, and the evil spirit departed from him.
20 And Jesse took an ass laden with bread, and a bottle of wine, and a kid, and sent them by David his son unto Saul.
21 And David came to Saul, and stood before him: and he (David) loved him (Saul) greatly; and he became his armourbearer.
22 And Saul sent to Jesse, saying, Let David, I pray thee, stand before me; for he hath found favour in my sight.
23 And it came to pass, when the evil spirit from God was upon Saul, that David took an harp, and played with his hand: so Saul was refreshed, and was well, and the evil spirit departed from him.
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But now Saul has his eye on David and Saul is going to try to kill David.
I Samuel 18
11 And Saul cast the javelin; for he said, I will smite David even to the wall with it. And David avoided out of his presence twice.
12 And Saul was afraid of David, because the LORD was with him, and was departed from Saul.
13 Therefore Saul removed him from him, and made him his captain over a thousand; and he went out and came in before the people.
14 And David behaved himself wisely in all his ways; and the LORD was with him.
15 Wherefore when Saul saw that he behaved himself very wisely, he (Saul) was afraid of him.
16 But all Israel and Judah loved David, because he went out and came in before them.
(That would cause Saul to hate and fear David even more.)
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I Samuel 18
21 And Saul said, I will give him her, that she may be a snare to him, and that the hand of the Philistines may be against him. Wherefore Saul said to David, Thou shalt this day be my son in law in the one of the twain.
(Saul kept sending David to fight the Philistines in hopes that the Philistines would kill David. But the Lord with with David and he was victorious against the Philistines.)
26 And when his servants told David these words, it pleased David well to be the king's son in law: and the days were not expired.
27 Wherefore David arose and went, he and his men, and slew of the Philistines two hundred men; and David brought their foreskins, and they gave them in full tale to the king, that he might be the king's son in law. And Saul gave him Michal his daughter to wife.
28 And Saul saw and knew that the LORD was with David, and that Michal Saul's daughter loved him.
29 And Saul was yet the more afraid of David; and Saul became David's enemy continually.
30 Then the princes of the Philistines went forth: and it came to pass, after they went forth, that David behaved himself more wisely than all the servants of Saul; so that his name was much set by.
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Saul sets it up for David to marry his youngest daughter and to fight Philistines.
25 And Saul said, Thus shall ye say to David, The king desireth not any dowry, but an hundred foreskins of the Philistines, to be avenged of the king's enemies. But Saul thought to make David fall by the hand of the Philistines.
27 Wherefore David arose and went, he and his men, and slew of the Philistines two hundred men; and David brought their foreskins, and they gave them in full tale to the king, that he might be the king's son in law. And Saul gave him Michal his daughter to wife.
28 And Saul saw and knew that the LORD was with David, and that Michal Saul's daughter loved him.
30 Then the princes of the Philistines went forth: and it came to pass, after they went forth, that David behaved himself more wisely than all the servants of Saul; so that his name was much set by.
And Saul continued as David's enemy and tried many times to catch David and to kill David. But God was with David.
2 Samuel 3:1 Now there was long war between the house of Saul and the house of David: but David waxed stronger and stronger, and the house of Saul waxed weaker and weaker.
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