Your Lord has decreed. This is a form of the word 'to announce' and it means to inform. The one who is determined about a matter talks to himself about it and informs it through his actions. It is akin to the act of swearing an oath, like 'the knowledge of Allah' and 'the witness of Allah'. Therefore, it is responded to in the manner of an oath, which is His saying: 'Certainly, He will send.' The meaning is: And when your Lord has decreed and written upon Himself that He will send against the Jews until the Day of Resurrection someone who will inflict upon them severe punishment. They used to pay tribute to the Magians until Allah sent Muhammad, blessings and peace be upon him, and imposed it upon them. It will remain imposed upon them until the end of time. The meaning of 'He will send against them' is that He will empower over them, as in His saying: 'We sent against you servants of Ours who are mighty in strength.'