Shamsuddeen Muhammad bin Abee Bakr Ibn Qayyim aj-Jawzeeyah (d.751) -Rahimullaah- said: ‘It is the statement due to which the heavens and the earth were created and all created beings were created due to it. The nations were built upon it and the Qiblah (the direction of prayer) was placed due to it.
The swords of Jihad were unsheathed due to it and it is the manifestation of the right of Allaah upon all of His slaves. Blood, wealth and offspring in this world are granted protection because of it, as it is a safeguard from the punishment of the grave and punishment of the fire.
The Statement of Tawheed is the proclamation without which no one can enter Paradise, and it is the rope without which it is not possible to reach Allaah. It is the statement of Islaam, and the key to the land of Salam (Peace).
And by it people are divided into wretched and happy and the accepted and the rejected. And with the Statement of Tawheed the lands of Kufr (disbelief) and the lands of Eemaan are separated, and the land of delight and the land of wretchedness and shame are distinguished from each other. And it is the pillar that carries that which is obligatory and that which is the Sunnah: ‘Whoevers last statement is ‘La ilaha ‘illa Allaah’ enters Paradise.’
[Taken from: ‘Loving Allaah’ by Ibn Qayyim aj-Jawzeeyah p.252]