It is, therefore, obvious that utterances of these words, the kalima, is a specific article of faith for every muslim; its recitation is the affirmation or testfication that is fundamental to Islam, without which one does not enter the fold of Islam. It is the first and the most important of the five pillars of Islam. The Holy Prophet SAAW said:
The sense of the shahadah is that one sees that only the Absolute,
only Allah, is Reality. The second shahadah, in defining the Prophet
Muhammad, also defines through him the relationship of menifestation or
creation as a message or revelation from Allah.
Although single uttering of the shahadah is enough, but to let
its realization penetrate one's core, the shahadah must be repeated
and lived to et it transform the individual.
The first shahadah casts man into an endless ocean, that of the Eternal, the Tremendous. The second, Muhammadun Rasoolu-Llah, in which Muhammad (saaw) is at once Prophet, Revelation and the Master Creation, is a unique Vessel coming from unique Reality, and thereby existing through It, which saves man from drowning in the infinite depths of the Absolute. (Cyril Glasse, The Concise Encyclopaedia of Islam.)