Is One Only a Kafir If He Knowingly Rejects the Truth?

This is not the case, as the below Qur’anic passages demonstrate:

Qur’anic verse 1: They are so ignorant that they don't know that they are the ones who are misguided.

وَإِذَا قِيلَ لَهُمْ آمِنُوا كَمَا آمَنَ النَّاسُ قَالُوا أَنُؤْمِنُ كَمَا آمَنَ السُّفَهَاءُ ۗ أَلَا إِنَّهُمْ هُمُ السُّفَهَاءُ وَلَٰكِن لَّا يَعْلَمُونَ

And when it is said to them, "Believe as the people have believed," they say, "Should we believe as the foolish have believed?" Unquestionably, it is they who are the foolish, but they know [it] not. [2:13]

Qur’anic verse 2: They assume that they know their scriptures.

وَمِنْهُمْ أُمِّيُّونَ لَا يَعْلَمُونَ الْكِتَابَ إِلَّا أَمَانِيَّ وَإِنْ هُمْ إِلَّا يَظُنُّونَ

And among them are unlettered ones who do not know the Scripture except in wishful thinking, but they are only assuming. [2:73]

Qur’anic verse 3: They think they are guided.

فَرِيقًا هَدَىٰ وَفَرِيقًا حَقَّ عَلَيْهِمُ الضَّلَالَةُ ۗ إِنَّهُمُ اتَّخَذُوا الشَّيَاطِينَ أَوْلِيَاءَ مِن دُونِ اللَّهِ وَيَحْسَبُونَ أَنَّهُم مُّهْتَدُونَ

A group [of you] He guided, and a group deserved [to be in] error. Indeed, they had taken the devils as allies instead of Allah while they thought that they were guided. [7:30]

Qur’anic verse 4: They think they are doing good.

الَّذِينَ ضَلَّ سَعْيُهُمْ فِي الْحَيَاةِ الدُّنْيَا وَهُمْ يَحْسَبُونَ أَنَّهُمْ يُحْسِنُونَ صُنْعًا

"Those whose efforts have been wasted in this life while they thought that they were acquiring good by their deeds!  [18:104]

Qur’anic verse 5: They do not know the truth.

أَمِ اتَّخَذُوا مِن دُونِهِ آلِهَةً ۖ قُلْ هَاتُوا بُرْهَانَكُمْ ۖ هَٰذَا ذِكْرُ مَن مَّعِيَ وَذِكْرُ مَن قَبْلِي ۗ بَلْ أَكْثَرُهُمْ لَا يَعْلَمُونَ الْحَقَّ ۖ فَهُم مُّعْرِضُونَ

Or have they taken for worship (other) aliha (gods) besides Him? Say: "Bring your proof:" This (the Qur'an) is the Reminder for those with me and the Reminder for those before me. But most of them know not the Truth, so they are averse. [21:24]

Qur’anic verse 6: They assume wrongly about the purpose of God’s creation.

وَمَا خَلَقْنَا السَّمَاءَ وَالْأَرْضَ وَمَا بَيْنَهُمَا بَاطِلًا ۚ ذَٰلِكَ ظَنُّ الَّذِينَ كَفَرُوا ۚ فَوَيْلٌ لِّلَّذِينَ كَفَرُوا مِنَ النَّارِ

And We did not create the heaven and the earth and that between them aimlessly. That is the assumption of those who disbelieve, so woe to those who disbelieve from the Fire. [38:27]

Qur’anic verse 7: They assume wrongly about God.

وَذَٰلِكُمْ ظَنُّكُمُ الَّذِي ظَنَنتُم بِرَبِّكُمْ أَرْدَاكُمْ فَأَصْبَحْتُم مِّنَ الْخَاسِرِينَ

And that was your assumption which you assumed about your Lord. It has brought you to ruin, and you have become among the losers." [41:23]

Qur’anic verse 8: They presumed that they have a leg to stand on.

يَوْمَ يَبْعَثُهُمُ اللَّهُ جَمِيعًا فَيَحْلِفُونَ لَهُ كَمَا يَحْلِفُونَ لَكُمْ ۖ وَيَحْسَبُونَ أَنَّهُمْ عَلَىٰ شَيْءٍ ۚ أَلَا إِنَّهُمْ هُمُ الْكَاذِبُونَ

On the Day Allah will resurrect them all, and they will swear to Him as they swear to you and think that they are [standing] on something. Unquestionably, it is they who are the liars. [58:18]