We strengthened his kingdom, and granted him (David) wisdom and... (Sād 38:20)

وَشَدَدْنَا مُلْكَهُ وَآتَيْنَاهُ الْحِكْمَةَ وَفَصْلَ الْخِطَابِ
We strengthened his kingdom, and granted him (David) wisdom and decisive speech (to inform, and convince, and lead). (Sād 38:20)

Right before this verse, God Almighty reminds our Prophet and everyone else through him what an excellent servant Prophet David, upon him be peace, was and mentions him with some of his other excellencies, as follows:

And remember Our servant David, powerful (in his glorification of God, in knowledge, in kingdom, and in fighting). Surely he was one ever turning to God in penitence. We subdued the mountains to glorify (their Lord) along with him in the afternoon and bright morning; and the birds assembled; all were turned to Him (in devotion and glorification). (Sād 38:17–19)

Following these two verses, God Almighty emphasizes that David set a praiseworthy example for combining kingdom with nearness to Him. Here, He draws the attention to the following three favors bestowed on David, upon him be peace:

“We strengthened his kingdom,” expresses that having suffered many afflictions, hardships, and calamities, Prophet David was made a Caliph in addition to his Messengership, and his administration was confirmed and strengthened. This also gives the implicit glad tidings that the future of Prophet Muhammad’s mission will be very bright.

“And We granted him wisdom,” which is an expression of a profound dimension of David’s Prophethood, implies that Prophet Muhammad, upon him be peace and blessings, would be favored with perfect wisdom.

“And (We granted him) decisive speech,” suggests that Prophet Muhammad, that leader of both humankind and jinn and the preacher of all times and places, upon him be peace and blessings, would be exceptional in effective and decisive speech. Just as the invocations and psalms of David echoed through mountains, the Qur’anic tunes of the Nightingale of Prophets, upon him be peace and blessings, would one day resonate through almost all breasts.

Many of the classical commentaries on the Qur’ān based on reports from the Prophet, Companions, and earliest scholars interpreted the expression of “fasla’l-khitāb” (decisive speech) as “ammā ba‘du” (after this); that is, the ability to separate two speeches from each other and pass from the former to the latter. However, almost everyone has this ability; therefore, it does not suit the eloquence of the Qur’ān and the status of Prophethood to mention such an ordinary ability as a great favor on a Prophet. So it means the ability to analyze and describe matters or truths so systematically and persuasively that everyone is satisfied and enlightened regarding the subject. It is also the ability to analyze and clarify a matter down to the minutest detail without causing any objections or further questions.