It will not benefit the one who says it unless the following seven conditions are fulfilled.
1. Knowledge (al-ilm) of its meaning, what it negates and affirms. If a person says it without knowing its meaning, nor what its requirements are, then he will not benefit by it. Because he has not believed in what it requires. Rather, he is like someone who speaks in a language that he does not understand.
2. Certainty (al-yaqeen) which is to have complete surety of it and which leads to repelling any doubt or suspicion about it
3. Exclusively worshipping Allah (al-ikhlaas) which is the contrary of shirk. This is what la ilaaha illallaah points towards.
4. Truthfulness (as-sidq) which prevents hypocrisy (nifaaq). Indeed, the hypocrites uttered it with their tongues, but did not inwardly believed in what it signified.
5. Love (al-mahabbah) for this kalimah and having love and pleasure for whatever it necessitates. This is contrary to the state of the hypocrites.
6. Submissive compliance (al inqiyaad), by fulfilling its rights which are the obligatory actions with sincerity to Alla and seeking His good pleasure. This is a requirement.
7. Acceptance (al-qabool), which prevents rejection. This is achieved by acting upon what Allaah has commanded and abandoning whatever He has prohibited.
*Scholars have deduce these conditions from those texts of the Book and the Sunnah which are pertinent to understanding this great kalimah; and which explain tis rights and restrictions and that it is not merely a word that a person utters.*
Taken from the book: THE DECLARATION OF FAITH, by Shaykh ibn Fawzaan al Fawzaan