Either you are FULLY OBEDIENT (Muslim) to Allah or you are UNJUST and unjust is in FIRE.

The Ayat: 72:14-15

72:14

وَأَنَّا مِنَّا الْمُسْلِمُونَ وَمِنَّا الْقَاسِطُونَ ۖ فَمَنْ أَسْلَمَ فَأُولَٰئِكَ تَحَرَّوْا رَشَدًا

“And among us are Muslims [in submission to Allah ], and among us are the unjust. And whoever has become Muslim – those have sought out the right course.”

72:15

وَأَمَّا الْقَاسِطُونَ فَكَانُوا لِجَهَنَّمَ حَطَبًا

“But as for the unjust, they will be, for Hell, firewood.”

The clear general meaning of the Ayah is, disbelieving Allah is an act of UNJUST.
Not recognizing one’s creator is UNJUST.
Being Kafir is UNJUST.
As if the Ayah clearly says, Either you are Muslim or you are Unjust.
That’s a clear statement about Kafir.

But then we should translate the meaning as per the impact of what it means in today’s context and today’s condition.

As for the word ‘Muslim’, what does it mean?
It means those who submit to Allah, those who obey Allah fully and full-heartedly.
Therefore, the Ayah should be a matter of concern to those namesake Muslims,
those who disobeyed Allah every day by not following Allah’s command to worship Him,
those who are not following the system of Allah in their life and
those who are following a system other than Allah’s system, the Deen presribed by Allah Ta’ala

So, in a traditional translation, the ‘modern Muslim’ of today, will actually find no impact of these Ayat because they think they are ‘Muslim’.
But if we take the literal meaning of Muslim properly (which is, fully obedient to Allah) and apply it then we should translate it like this, 
“Not FULLY OBEDIENT to Allah is an act of UNJUST and unjust means FIRE”
so that namesake Muslims might give it a thought InShaaAllah.

And Allah knows best.
May Allah guide Muslim and make us fully submiting to Allah.