The first message of islam to call people to.

 The first message of islam to call people to. 

Alhamdulillah, the first knowledge of Islam to call a person to is the message of Tawheed. 

The message of all messengers which Allah sent them to. 

Allah is one and only without a beginning or end. 

Allah says in Quran, 

An-Nahl 16:36

And We certainly sent into every nation a messenger, [saying], "Worship Allāh and avoid ṭāghūt."[1] And among them were those whom Allāh guided, and among them were those upon whom error was [deservedly] decreed. So proceed [i.e., travel] through the earth and observe how was the end of the deniers.

The taghut here scholars say it is shirk. 

The message of tawheed is the purpose why all messengers were sent and 3/4 of the Quran talks of oneness of Allah and to worship him alone. 

Allah s.w.t before creation gathered all the soul and made them to bear witness he is the lord.

Allah says, 

Al-A'raf 7:172

And [mention] when your Lord took from the children of Adam - from their loins - their descendants and made them testify of themselves, [saying to them], "Am I not your Lord?" They said, "Yes, we have testified." [This] - lest you should say on the Day of Resurrection, "Indeed, we were of this unaware."

So our soul bear witness that Allah is the lord before we came to this world.

This is the greatest of testimony between Allah and his creations.