It is reported on the authority of Ibn Abbas that Muadh said: The Messenger of Allah (peace and blessings of Allah be to him) sent me (as governor of Yemen) and (at the time of departure) instructed me thus: you will soon find yourself in a community one among the People of the Book; so first call them to testify that there is no god but Allah, that I (Muhammad) am the messenger of Allah, and if they accept this, then tell them that Allah has enjoined upon them five prayers during the day and the night, and if they accept it, then tell them that Allah has made Zakat obligatory for them that it should be collected from the rich and distributed among the poor, and if they agree to it, don’t pick up (as a share of Zakat) the best of their wealth. Beware of the supplication of the oppressed for there is no barrier between him and Allah.
(Sahih Muslim)
The People of the Book never denied the existence of God. Their belief however was not correct as it was alloyed with wrong conceptions and mistaken notions. This shows that mere faith in God is not enough. Therefore, the Holy Messenger asked Muadh to call the People of the Book to testify God, first and foremost. Calling to the prophethood of Muhammad is indicative of the fact that belief in his prophethood is an integral part of the faith.
This hadith highlights the practical wisdom with which Dawah should be imparted to the fellow beings.