The following is from John’s commentary for John 5:3.
As the Holy Spirit, dwelling in the Prophets, was a sufficient witness of the divine presence, religion at that time needed no other confirmation; for the Law had been sanctioned by abundantly sufficient miracles, and God ceased not to express, by innumerable testimonies, his approbation of the worship which he had enjoined. But about the time of Christ’s coming, as they were deprived of the Prophets and their condition was very wretched, and as various temptations pressed upon them on every hand, they needed this extraordinary aid, that they might not think that God had entirely left them, and thus might be discouraged and fall away. For we know that Malachi was the last of the Prophets, and, therefore, he closes his doctrine with this admonition, that the Jews may “remember the Law delivered by Moses,” (Mal 4:4) until Christ appear. God saw it to be advantageous to deprive them of the Prophets, and to keep them in suspense for a time, that they might be inflamed with a stronger desire for Christ, and might receive him with greater reverence, when he should be manifested to them. [source]