Why was our master David called the Caliph of God?

The successorship of David (peace be upon him) is affirmed by the Quranic text, and its meaning is that he was a successor to those who preceded him among creation, or for implementing Allah's rulings. A caliph (khalifah) is one who succeeds another or is made a successor. It does not mean that he is a successor to Allah. Indeed, many scholars have prohibited bestowing the title "Caliph of Allah" upon Muslim rulers. Rather, one says: "The Caliph," or "Caliph of the Messenger of Allah," or "Commander of the Faithful." The first to be called "Commander of the Faithful" was Umar ibn al-Khattab (may Allah be pleased with him).