A person becomes a Muslim by testifying to the truth of the statements: There is none worthy of worship except Allah and Muhammad is the Messenger of Allah. Since this is a testimony or bearing witness to the truth of something, it must be a “public proclamation” (or, in other words, not something hidden within oneself but, instead, conveyed to others).
Ibn Abu al-Izz wrote, [The Prophet (peace be upon him)] has made it absolutely clear that a person is definitely not a believer if it is claimed that he believes in the Prophet (peace be on him) but he does not profess it with his tongue, even though he can...
This profession of the tongue plays a three-fold role. It is first a statement of a fact. One is testifying that he recognizes the truthfulness of that statement of faith. This would be analogous to a person giving testimony in a court of law. All he is really stating is that those are the facts that he believes to be true.
Second, though, it is statement of commitment to that fact. It is an admission by the person that he intends to adhere to the requirements and guidance of what he has testified to.
Third, it is a public proclamation that the individual has now joined the fold of Muslims, accepting all of the rights and responsibilities that such implies.
“There is none worthy of worship except Allah.”“I testify that none is worthy of worship except Allah and…,”—and everyone should be extremely concerned over whether his testimony of faith is acceptable to Allah or not.
A study of the verses of the Quran and the hadith of the Prophet (peace be upon him) will show that there are a number of conditions for the soundness of one’s testimony of faith. Again, it is important that every Muslim verify that he is meeting these conditions in his own life with respect to his own testimony of faith. The new convert should consider these conditions concerning his testimony of faith. Preferably (but not necessarily), these would have been explained to him before he undertook the declaration of faith.
Similarly, the Messenger of Allah (peace be upon him) said
No one meets Allah with the testimony that there is none worthy of worship but Allah and I am the Messenger of Allah, and he has no doubt about that statement, except that he will enter Paradise. 
Recorded by Muslim
Allah says,“They alone seek leave of you [not to participate in Jihad] who believe not in Allah and the Last Day and whose hearts feel doubt, so in their doubt they waver
This condition also means that the Muslim believes in whatever is stated in the Quran or stated by the Prophet (peace be upon him), without any right to choose what he wants to believe and what he wants to reject.
Allah says in the Quran, “It is not for a believing man or believing woman, when Allah and His Messenger have decreed a matter, to have any option in their decision. And whoever disobeys Allah and His Messenger has indeed strayed in plain error”
An eighth condition is that the person who states the testimony must deny every other object of worship. Although that is clear in the words of the testimony of faith, it does not seem clear to everyone who makes that testimony. Therefore, it needs to be mentioned explicitly.
- A possible exception to this would be when one fears immediate death upon displaying one’s Islam. Otherwise, one is expected to openly profess Islam even if he may face some opposition or hardship along the way. Such was the example of the Prophet (peace and blessings of Allah be upon him) and his Companions.
- Ibn Abu al-Izz, vol. 2, pp. 471. The translation used here is a pre-publication copy of Muhammad Abdul Haq Ansari, trans., Commentary on the Creed of at-Tahawi by ibn Abi al-Izz (Riyadh: Ministry of Higher Education, 2000). For the sake of the page numbering, the published version from Mu’assasat al-Risaalah will be the work referred to.
- The conditions of the testimony of faith are well-known and discussed in numerous works. See, for example, Haafidh ibn Ahmad Hakimi, Ma’arij al-Qubool bi-Sharh Sullim al-Wusool ila Ilm al-Usool fi al-Tauheed (Beirut: Dar al-Kutub al-Ilmiyah, 1983), vol. 1, pp. 307-315; Abdullah ibn Jibreen, al-Shahadataan (no city or publisher given, 1990), pp. 77-86; this author also presented most of this material on the two parts of the shahaadah in his The Friday Prayer: Part II: Khutbahs (I) (Aurora, CO: IANA, 1994), pp. 4-19; The Friday Prayer: Part III: Khutbahs (II) (Ann Arbor, MI: IANA, 1995), pp. 35-42