Allah sends Whomever He wills with the Message of Tawhid
(He sends down the angels with the Ruh) refers to the revelation. This is like the Ayat:
(And thus We have sent to you a Ruh (revelation) by Our command. You knew not what is the Book, nor what is the faith. But We have made it a light by which We guide whomever We will among Our servants.) 43:52
(to those servants of His whom He wills) meaning the Prophets, as Allah says:
(Allah best knows where to place His Message.) 6:124
(Allah chooses Messengers from angels and from men.) 22:75
(He sends the Ruh (revelation) by His command to whoever among His servants He wills to, that he may warn of the Day of Meeting. The Day when they will (all) come out, nothing about them will be hidden from Allah. Whose is the kingdom this Day: It is Allah’s, the One, the Irresistible!) (40:15-16)
((saying): “Warn…”) meaning that they should alert them.
(that none has the right to be worshipped but I, so have Taqwa of Me.) means, `fear My punishment, if you go against My commands and worship anything other than Me.’