Tawheed is the life of Hearts
Shaykh Khalid ad-Dhafiri said:
‘Life is given to the hearts with Tawheed. A real life.
So what is this real life with?
With Tawheed, this is why Allaah -Azza wa Jal- resembled the people of Tawheed and the people of Eemaan with having life.
What did Allaah resemble the people of Shirk with? With being dead.
As Allaah -Azza wa Jal – said:
يَا أَيُّهَا الَّذِينَ آمَنُوا اسْتَجِيبُوا لِلَّـهِ وَلِلرَّسُولِ إِذَا دَعَاكُمْ لِمَا يُحْيِيكُمْ [الأنفال:24
《 O you who believe! Answer Allah (by obeying Him) and (His) Messenger when he calls you to that which will give you life 》[Anfal: 24]
http://miraath.net/ar/content/makalate/التوحيد-حياة-القلوب