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yā-ʾahla l-kitābi qad jāʾakum rasūlunā yubayyinu lakum ʿalā fatratin mina r-rusuli ʾan taqūlū mā jāʾanā min bashīrin wa-lā nadhīrin fa-qad jāʾakum bashīrun wa-nadhīrun wa-llāhu ʿalā kulli shayʾin qadīrun
O People of the Book! Certainly Our Apostle has come to you, clarifying [the Divine teachings] for you after a gap in [the appearance of] the apostles, lest you should say, ‘There did not come to us any bearer of good news nor any warner.’ Certainly, there has come to you a bearer of good news and a warner. And Allah has power over all things.
— Ali Quli Qarai
Surah Al-Ma'idah, Verse 19
The intermission between the time of Hadrat Messiah (Jesus Christ) (as) and the advent of Prophet Muhammad (S) lasted about six hundred years. During the time when there exists not an appointed Divine prophet, the globe is not empty of Allah's Authority, because the prophets' successors have always existed. As the words of Amir-ul-Mu'mineen Ali (as) indicates, the earth never lacks godly authority whether he has power, or not, because the way of Allah should not be concealed for those who want to pave it9.
Hence, the existence of the time of intermission between coming one prophet after the former does not mean that people are left alone to themselves.
By the way, the intermissions, whether they are long or short, are certainly useful in the system of divine educational arrangements. There are several examples of this meaning in the length of the history of prophets. For instance; the separation of Moses (as) from people, the spiritual retirement of prophets, the cease of revelation unto the Prophet Muhammad (S), and minor occultation and major occultation are some distinguished references for the idea.
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