In fact, the shahadah itself is a testimony. When one testifies to something, one must know what it is that he is testifying concerning. Obviously, a testimony about something that one does not have any knowledge about is absolutely unacceptable. Allah says in the Quran, "Save him who bears witness unto the truth knowingly" (Qur'an, al-Zukhruf 86).
Now this condition probably sounds obvious. If someone said to you, "There is no God except Allah," and then said that what he meant by Allah was Jesus, you would immediately tell him that this shahadah is nonsense. Yet can you imagine that there is a Muslim country in the world that until recently used to have a yearly week-long celebration to "the Gods of the Sea"! All along they call themselves Muslims and they make shahadah numerous times a day. This clearly shows that they have no idea what the meaning of their shahadah is. Do you think that this kind of shahadah will open the doors to Paradise for them?
Today, there are many Muslims who wonder why we should not accept secularism. They think that there is nothing wrong with secularism! Many of these Muslims pray five times a day and utter the shahadah repeatedly. Yet they see nothing wrong with accepting a Lawgiver other than Allah? What kind of shahadah are these people making?
All of us do our best to learn at least the basics of Islamic beliefs. The basics of what shahadah means. In this way, Allah willing, we will be making the correct shahadah. We will be witnessing to the truth as we are supposed to be witnesses to it.
The second condition of the shahadah is certainty (or al-yaqeen). This is the opposite of doubt and uncertainty. In Islam, in fact, any kind of doubt is equivalent to Kufr or disbelief. We must, in our hearts, be absolutely certain of the truth of the shahadah. Our hearts must not be wavering in any way when we testify to the truth of, "There is none worthy of worship except Allah."
Allah describes the believers in the Quran as those who have belief in Allah and then their hearts waver not. Allah says, "The (true) believers are those only who believe in Allah and His messenger and afterward doubt not, but strive with their wealth and their lives for the cause of Allah. Such are the sincere" (Qur'an, al-Hujurat 15). 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 hat statement, except that he will enter Paradise." (Recorded by Muslim.)
Indeed, Allah describes the hypocrites as those people whose hearts are wavering. For example, Allah says, "They alone seek leave of thee (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" (Qur'an, al-Tauba 45).
Many scholars have stated that the diseases of the heart, or the doubt and suspicions that one allows into one's heart, are more dangerous for a person's faith than lusts and desires. This is because lusts and desires may be satisfied at some time yet the person still knows them to be wrong and he may control himself and repent and give up that evil deed. On the other hand, the doubts and suspicions may linger in the heart, with no cure, until the person finally leaves Islam entirely or continues to practice Islam while, in fact, in his heart he does not have the true faith.
The greatest cure for these doubts is seeking knowledge. It is through sound knowledge of the Quran and the sunnah that most of these doubts will be removed. By study and understanding, one will attain certainty. And as one studies and learns more, his certainty will be made firmer and firmer.
I will give you just one example on this point. This is related to all of the doubts, suspicions and misconceptions that are circulating about the authenticity of the hadith literature. For example, some Muslims even claim that hadith were not recorded until at least 200 years after the death of the Prophet (peace be upon him). Indeed, there are Muslims who seem to have a great deal of doubt about hadith and are very quick to reject hadith on that basis. But, in fact, if one actually takes the time to study the history and preservation of hadith, one will immediately recognize that all those claims and charges against the hadith are not based on anything scientific whatsoever. They are nothing but false claims that come from Satan and which Muslims with poor understanding and knowledge have allowed to creep into their hearts.
Let me make one last point about this condition of certainty. As I alluded to earlier, doubts and misconceptions are very dangerous for a person's faith. Doubt and wavering are tantamount to apostasy in Islam, as I mentioned earlier. Hence, every Muslim should do best to safeguard himself from such doubts and remain away from the sources of doubts- especially if they are not well grounded in the Islamic sciences and do not have the the knowledge to refute such doubts and misconceptions. Hence if a person has an associate or friend, even if he be a Muslim friend, who is always making him doubt Allah or the religion, then he should remain away from that person in order to safeguard his own religion. Many Muslims nowadays study courses on Islam taught by Orientalists and because of their weak backgrounds in Islamic studies, they are influenced by the nonsense that some of these Orientalists are teaching in the name of "science".
Similarly, many brothers and sisters nowadays are spending hours at a time with the computer news groups and bulletin boards. A lot of nonsense and falsehood are posted on those news groups and bulletin boards. Again, the one who is not very knowledgeable of his religion may easily be influenced and affected by the misconceptions and false arguments that he reads in those bulletin boards. He should remain away from those things and get a deeper knowledge of Islam through the authentic sources of the religion.
Again, the greatest counterforce that can defeat doubts and misconceptions, after the bounty and guidance of Allah, is sound knowledge and understanding of the religion. When a person has that, he will not be affected by the weak arguments put out by the enemies of Islam and he will be, Allah willing, from among those described in the Quran, " It is only those who have knowledge among His slaves that fear Allah" (Qur'an, Fatir 28)