He Stresses That God Has Already Redeemed Him Once (Psalms 55:18-19).
It would appear from this that an attempt had already been made to get rid of him by an armed assault. As one of Saul's commanders he would, of course, have had men who were loyal to him, thus the attempt would have had to be made by a good number of men. It was possibly this large contingent, which he had either driven off or defeated, which had been led by his one-time friend. If the situation in life was of the time when David was under threat from Saul, while still acting as one of his commanders, it was clearly at a stage when Saul did not feel that he could act openly against one of the people's favourites, with the consequence that his attempts had to be made surreptitiously. It could be that his erstwhile friend, with a group of willing soldiers, had tried to enter his house at night, or that they had tried to catch him unawares by subtlety in some lonely place when he was away from the city on a military mission. Indeed his ‘friend' might have used his friendship in order to lure him into danger. One possible instance of this was 1 Samuel 19:11-17.
‘He has redeemed my life in peace from the battle which was against me,
For they were many (who strove) with me.'
David confidently asserts how God had ‘redeemed his life in peace' (delivered him safely), when a goodly number of men had come to do battle against him. We can understand why, if the group was led by his supposed friend, he had received such a shock. There is no verb in the second clause and one thus has to be read in. But the point is that with God's help he had survived the attack on his life.
‘For there were many with me' might indicate that the attempt had failed because he himself at the time was able to call on his men, but it more probably signifies men who were ‘battling with me'.
‘God will hear, and answer them,
Even he who abides of old, [Selah
Those who have no changes,
And who do not fear God.'
‘God will hear' probably refers to David's prayers (alternately it could mean that He would hear their false calumnies and their plots against David). But the ones who will be answered are his enemies. God will provide a full answer to their accusations and attempts on David's life, by saving David and bringing judgment on them. For He is the One Who is from of old, and has always in the past proved faithful to His own. Note the emphasis on God as continually active in the past, something which is drawn to men's attention by a pause in the music (selah).
The ones who will be answered are those who ‘have no changes and who do not fear God'. The word for ‘changes' usually refers to changes of clothing, but in Job 10:17 it seemingly refers to changes of circumstances or changing troops (‘changes and a host'). It probably here signifies ‘no changes of mind and attitude' (they are obstinate in the performance of their evil task). But it could mean that it was always the same men who made attempts on his life. That they do not fear God (among other things they ignore the fact that David is God's anointed) indicates that they are unscrupulous and ready to do anything that is required of them without a twinge of conscience.
The purpose of the musical pause may well be in order that the hearers might for a brief moment concentrate their thoughts on the permanence of God.