the most profoundly revered sacred formula: La ilaha illa-Allah (also can be written as La ilaha illallah) in exoteric interpretation means: there is no god but One God.
that is for bringing the whole of humanity into oneness and to remove all differences. differences in the name of God has to be removed, that was the goal of this message in all ages. an expression to say to all of humanity that whatever name and form you are calling God, the Essence is One.
in esoteric meaning, the sacred formula contains the Secret of Secrets. Then it reveals something truly surreal, something which by the very nature of phenomenal world makes it very hard to realize. it's something which endless sages after sages came to realisation; some after spending a whole life time, some after forty years of retreat, some after very diligent spiritual practice and scholarship. yet they all understood this.
and that meaning is profound yet simple:
there is nothing but God
there is no thing but God
there is not-a-thing but God
there is no duality but Supreme Unity
there exists no duality but Oneness
this is the ultimate teaching of all sacred path and faith, including that of great teaching of Advaita (not two or many but One, non-Dual) of Vedanta in Hinduism. you look at the teachings of Buddha, you will find all he was working on the 'nothing' part of it. you look at the christian cross and you will find the cancelling out of the little 'i', the self. that's cancelling out the exact root of duality. only after the supreme realization of this secret Jesus could proclaim, 'i and father are One'.
in Quran even after so much emphasis on Oneness, God calls Himself We, why?!
esotericaly this is because everything is His Essence. This 'everything' and 'We' is His, is nothing but He.
wheversoever you turn you see the Face of God.
the sacred formula holds this Ultimate Secret, that is why Prophet Muhammad said that on one side of the Divine scale if you put this sacred formula and on the other side everything that is in the creation, yet La ilaha illaAllah will weigh more in terms of what it beholds inside of it.