The Prophet (salal-laahu-alayhi-wasallam) called the Ummatud Da’wah to that which is obligatory for today’s Ummatul Ijaabah to call to!
In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Shaikh Raslaan (may Allaah preserve him) said:
The Prophet (sallal-laahu-alayhi-wasallam) was sent within circumstances concerning which a Muslim cannot say: ”Indeed, the circumstances in which we live in more evil and more deprave in its depravity than (the circumstances in which the Prophet was sent).” (That is) because the Prophet (salal-laahu-alayhi-wasallam) was sent to a disbelieving people and he guided them through one path-to worship one Lord, one 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. the creator, provider and controller of the affairs of the creation) besides Him.
Therefore, the path [of Prophet-salal-laahu-alayhi-wasallam in calling to Allaah in those circumstances] was one, and it is the path that the Messengers brought. The Prophet (salal-laahu-alayhi-wasallam) called the Ummatud Da’wah (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.
[Source: For further details see: Mataa Ta’oodu Ilaynaa philisteen’ pages 12-13]