‘O you who have believed, believe!’

يَا أَيُّهَا الَّذِينَ آمَنُوا آمِنُوا بِاللَّهِ وَرَسُولِهِ وَالْكِتَابِ الَّذِي نَزَّلَ عَلَىٰ رَسُولِهِ وَالْكِتَابِ الَّذِي أَنزَلَ مِن قَبْلُ ۚ وَمَن يَكْفُرْ بِاللَّهِ وَمَلَائِكَتِهِ وَكُتُبِهِ وَرُسُلِهِ وَالْيَوْمِ الْآخِرِ فَقَدْ ضَلَّ ضَلَالًا بَعِيدًا

O you who have believed, believe in Allah and His Messenger and the Book that He sent down upon His Messenger and the Scripture which He sent down before. And whoever disbelieves in Allah , His angels, His books, His messengers, and the Last Day has certainly gone far astray. [Surah Nisa, verse 136]

I found it strange that Allah SWT is addressing the believers (O you who have believed) and then commanding them to ‘Believe’, ‘Have eman’ again!

Wondering what the significance of this was, I referred to Ustadha Taymiyah Zubairs Talim al Quran course to understand the following:

This command of Allah SWT is understood in two ways:

To have firmness in eman. A person becomes firm in eman, continuously maintaining his eman in order to be steadfast and strengthen his faith.
It is common that our eman fluctuates. At one point our salah, zikr etc is so good and at other times its difficult to even focus in salah.
So Allah swt is commanding us to continuously persevere in Eman; remain steadfast in eman. And to be firm in all the branches of eman; not focusing on only one aspect and forgetting other aspects like focusing on the heart and forgetting the actions or focusing on the actions and forgetting the heart.
To complete our faith. In other words, those of you that claim to believe, make that belief a reality. Prove your faith. Saying ‘I’m a mu’min, I’m a believer’ is not enough. One has to prove it in his actions. SHOW our eman and demonstrate our eman. Show the manifestations of the true level of eman in our lives.

What does it mean to believe in Allah SWT? First of all in His existence, then secondly in His Uluhiyyah (He is the ONLY One worthy of worship), thirdly in His Rububiyyah (that He is the Rabb, the creator), and fourthly in His Sifaath (in His names and attributes). If a person denies even one of these, then his eman is incomplete.

What does it mean to have belief in the messenger? [ Here رَسُولِهِ refers to the Prophet SAW because its singular. ]

Belief that he is haqq, the truth. He was a man, and not an angel etc. And that he really existed.
Belief that whatever the messenger brought, is true -> belief in the messenger entails belief in the book, his commands, his prohibitions and the miracles that he has brought and presented to people.
Belief in him entails obeying and following the Prophet SAW.

‘Belief in the book’ Which book? The Qur’an.

Belief that it is from Allah swt.
Belief that whatever is in the book is truth. Every single statement of the Quran is haqq.

‘Belief in all of the books that was revealed before’: Even if you didn’t have their original versions with you, Still believe that Allah swt is the One who revealed those books.

‘And one who disbelieves in Allah SWT (deny His sifah etc) and His angels, His books, His messengers, and the Last Day has certainly gone far astray.’ : He has gone faaar away from the truth, the person who disbelieves in any of what is mentioned here has strayed.

Lessons:

It is not enough to just say ‘I believe’. A person must constantly strive to improve their Iman. People who love clothes and fashion, do they go shopping only once a year? No. They regularly go to see whats new, or if there’s a sale etc etc. They make sure they aren’t ‘lagging behind’.
When we like something, we are going to try to improve it. Its not possible that we will ignore it. So having Iman is not just the claim, but to constantly try to strengthen and improve that iman.
Belief in all of the above mentioned is eman. Not believing in one and disbelieving in the other.