But the sons of Belial shall be all of them as thorns thrust away, because they cannot be taken with hands:

Wesley's Notes

23:6 But - Having in the foregoing verse s described the nature, and stability of that kingdom which God had by a sure covenant settled upon him and his seed; and especially, upon the Messiah, who was to be one of his posterity; he now describes the nature and miserable condition, of all the enemies of this holy and blessed kingdom. As thorns - Which men do not use to handle, but thrust them away. And so will God thrust away from himself, and from his people, and kingdom, all those who shall either secretly or openly set themselves against it.

Jamieson-Fausset-Brown Bible Commentary

6. But the sons of Belial shall be all of them as thorns-that is, the wicked enemies and persecutors of this kingdom of righteousness. They resemble those prickly, thorny plants which are twisted together, whose spires point in every direction, and which are so sharp and strong that they cannot be touched or approached without danger; but hard instruments and violent means must be taken to destroy or uproot them. So God will remove or destroy all who are opposed to this kingdom.

Matthew Henry's Concise Commentary

23:1-7 These words of David are very worthy of regard. Let those who have had long experience of God's goodness, and the pleasantness of heavenly wisdom, when they come to finish their course, bear their testimony to the truth of the promise. David avows his Divine inspiration, that the Spirit of God spake by him. He, and other holy men, spake and wrote as they were moved by the Holy Ghost. In many things he had his own neglect and wrong conduct to blame. But David comforted himself that the Lord had made with him an everlasting covenant. By this he principally intended the covenant of mercy and peace, which the Lord made with him as a sinner, who believed in the promised Saviour, who embraced the promised blessing, who yielded up himself to the Lord, to be his redeemed servant. Believers shall for ever enjoy covenant blessings; and God the Father, Son, and Holy Ghost, shall be for ever glorified in their salvation. Thus pardon, righteousness, grace, and eternal life, are secured as the gift of God through Jesus Christ. There is an infinite fulness of grace and all blessings treasured up in Christ, for those who seek his salvation. This covenant was all David's salvation, he so well knew the holy law of God and the extent of his own sinfulness, that he perceived what was needful for his own case in this salvation. It was therefore all his desire. In comparison, all earthly objects lost their attractions; he was willing to give them up, or to die and leave them, that he might enjoy full happiness, Ps 73:24-28. Still the power of evil, and the weakness of his faith, hope, and love, were his grief and burden. Doubtless he would have allowed that his own slackness and want of care were the cause; but the hope that he should soon be made perfect in glory, encouraged him in his dying moments.