This is the message of peace, the message of salvation. It is the message of liberation from being enslaved to mundane things that keep shackling souls with worries and sorrows.
This is a call from the One Who controls everything, calling you, humankind, to submit!
Submit your life to Him and be calm. Make your motto of life La ilaha illa Allah (There is no god but Allah). By this motto, you testify to your incapacity as a creature provided with limited faculties. You testify that everything in the universe belongs to Him, and whatever He wants will come to pass in the way and the time He wants. You testify to your being created by Him in the way He wants:
{He it is Who shapes you in the wombs as He likes; there is no god but He, the Mighty, the Wise.} (Aal `Imran 3:6)
But despite His absolute power to do everything in anyway He likes, what we notice is that out of His infinite mercy He made everything perfect, created you and all what you see around you in the most beautiful way that we all behold:
{He has created the heavens and the earth in just proportions, and has given you shape and made your shapes beautiful: And to Him is the final Goal.} (At-Taghabun 64:3)
{No want of proportion will you see in the Creation of (Allah) Most Gracious. So turn your vision again: See you any flaw? Again turn your vision a second time: (Your) vision will come back to you dull and discomfited, in a state worn out.} (Al-Mulk 67:3-4)
Due to His mercy, the abnormalities are subject to the law of rarity; that is, ever since the creation of Heaven and earth and the creation of Adam, up to the present day, there is not any irregularity in the pattern of creation, and whatever few abnormalities that we see are also subject to His divine wisdom.
Despite all the constant recklessness of humankind, painful consequences never occur in a proportional measure, subhan Allah (glory be to Allah)! Compared to the high rate of speed-limit violations, how many accidents occur daily? Very few! Actually, His mercy exceeds His wrath.
{Whatever misfortune happens to you is because of the things your hands have wrought, and for many (of them) He grants forgiveness.} (Ash-Shura 42:30)
Another point here is, your testifying to Allah’s almighty power is an explicit recognition of your limit as a human being. By pronouncing the words “la ilaha illa Allah,” you are actually surrendering your limited power to the mighty Lord of unlimited power; you submit your will to Him. This is Islam.
But this attestation can never be genuine if it is not accompanied with love, because faith without love is dead. If you do not love Him, you will not genuinely obey Him. This is different from relationships between mortals where hypocrisy prevails, making it hard to distinguish between genuine and fake love. But with Almighty Allah, everything is transparent, so when we talk about this highest degree of love, we talk about something that is not tainted with any hypocrisy or deceit.
So the true and genuine Shahadah in Islam is the one coated with love: loving Allah, which of course is the sublime love. This love will automatically make it easier to show full obedience to His commands:
{Say: If you love Allah, then follow me, Allah will love you and forgive you your faults} (Aal `Imran 3:31).
Thus, you will find yourself spontaneously pronouncing the phrase “Muhammad Rasul Allah” (Muhammad is the Messenger of Allah) which is the second part of Shahadah in Islam.
Then bit by bit, you will find yourself on a very smooth spiritual path whereby the relationship you have developed with Allah will direct light into your heart, melting away darkness. Thus the phrase “la ilaha illa Allah” will echo in all what you do, what you see, what you hear, etc.
That is, it will develop into a conviction in your innermost self, reassuring you that it is through His command and wishes that everything happens. Hence this conviction is iman (faith) reposing in your heart. Henceforth your manners will be in proportional compatibility with that faith. That means you have accepted Allah into your life.