
What's interesting about these warriors that joined up with David was that they were defectors!
They changed loyalties!
They transferred their allegiance!
Most or all of them were formerly followers of King Saul, even Saul's relatives.
There were ambidextrous archers from the tribe of Benjamin (vs. 2-7) and desert warriors from the tribe of Gad that crossed the Jordan River to join David (vs. 8-15).
And when even more Benjamites defected, David was reluctant to trust them until they swore allegiance to him under the inspiration of the Spirit of God (vs. 16-18).
Men as far away as Manasseh defected and joined David's army (vs. 19-21).
All these defectors were skilled and brave warriors. They defected to David because they believed David was God's chosen king and they wanted to join him in his cause.
So the numbers of defectors grew until they became a great army. And, this army of defectors became the army of God!