Shaikh Raslaan (may Allaah preserve him) said:
The Prophet (sallal-laahu-alayhi-wasallam) was sent (during) circumstances about which a Muslim cannot say, ”Indeed, the circumstances in which we live in (today) are eviler and more depraved in its depravity than (the state of affairs in which the people lived when the Prophet was sent).” (This is) because the Prophet (salal-laahu-alayhi-wasallam) was sent to a disbelieving people and he guided them through one way. He directed them towards one path—the worship of one Lord; the only true God; The Perfect Lord and Master upon Whom all the creation depends and He is self-sufficient; The Unique One in His Names, Attributes and Actions. There is neither another true God (worthy of worship) besides Him nor another true Lord (i.e. Creator, Provider and Controller of the affairs of the creation) besides Him.
So the path [of Prophet (Salal-laahu-alayhi-wasallam) in calling to Allaah in those circumstances] was one, and it was the (path) the Messengers came with. The Prophet (salal-laahu-alayhi-wasallam) called the Ummatud Dawah (i.e. the nation that had not yet accepted the message of Islaam) to that which is obligatory for today’s Ummatul Ijaabah (i.e. the nation that has already accepted the call of Islaam) to call to.
[For further details see: Mataa Ta’oodu Ilaynaa Falasteen: page: 12-13]