Marriage is not merely a social agreement. Allah describes it in the Qur'an as a firm commitment.
وَأَخَذْنَ مِنكُم مِّيثَاقًا غَلِيظًا
"And they have taken from you a firm commitment." (Surah An-Nisā' 4:21)
A man enters this commitment by saying the words of acceptance during the Nikah:
قَبِلْتُ النِّكَاحَ (Qabiltu an-Nikāḥ) — "I accept the marriage."
With these few words, his life changes. He becomes a husband, and the woman becomes a wife. From that moment onward, he carries a new identity. His actions, choices, relationships, and responsibilities are guided by that sacred contract. He does not give another woman the place and rights that belong only to his wife because he knows he has entered into a binding relationship. Even if few mistakes or wrongs happen, they are forgiven. As long as this commitment is honoured, the marriage remains intact.
Now reflect on the Kalimah:
أَشْهَدُ أَنْ لَا إِلَٰهَ إِلَّا اللَّهُ وَأَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اللَّهِ
"I bear witness that there is no deity worthy of worship except Allah, and I bear witness that Muhammad is the Messenger of Allah."
This is an even greater commitment—between the servant ('abd) and his Lord (Rabb). A person enters Islam by pronouncing the Kalimah, but the Kalimah is not merely something to be recited once. It is a commitment to be lived. A Muslim lives within the Kalimah every hour of every day. His worship, love, fear, hope, trust, obedience, intentions, and actions should all remain faithful to this commitment.
Just as a faithful husband avoids anything that violates his marriage by giving another woman the rights that belong to his wife, a true believer avoids anything that violates the Kalimah by giving to anyone else the rights that belong only to Allah. That is the essence of Tawḥīd.
The continuous nature of the Kalimah becomes even clearer when we look at the Five Pillars of Islam in reverse. Note that five pillars of Islam are all actions.
Hajj is obligatory only once in a lifetime, and only for those who are physically and financially able.
Zakat is obligatory once every year, and only upon those who possess the required wealth.
Fasting is obligatory for one month every year, with concessions for those who are unable.
Ṣalāh is obligatory five times every day upon every accountable Muslim.
The Shahādah, however, is different. It is not once in a lifetime, once a year, one month every year, or five times a day. It is twenty-four hours a day, seven days a week, throughout one's entire life. A Muslim lives a life within this Sacred Kalima.
Hence without the Kalimah, the remaining pillars have no foundation. It is not simply words spoken by the tongue. It is a commitment believed by the heart, affirmed by the tongue, and manifested through every action until the servant returns to Allah.
Allah knows best.