(… السَّبِيلَ… ) (…the way…): It means the straight path which is to worship Allah (swt) Alone. He (swt) says:

وَأَنَّ هَٰذَا صِرَاطِي مُسْتَقِيمًا فَاتَّبِعُوهُ ۖ وَلَا تَتَّبِعُوا السُّبُلَ فَتَفَرَّقَ بِكُمْ عَن سَبِيلِهِ ۚ ذَٰلِكُمْ وَصَّاكُم بِهِ لَعَلَّكُمْ تَتَّقُونَ

(And, [moreover], this is My path, which is straight, so follow it; and do not follow [other] ways, for you will be separated from His way. This has He instructed you that you may become righteous.) (Al-An’am: 153)

The Path of Allah (swt) is the straight one. The curved path lengthens the distance for the person, as the straight line is the shortest way to reach the aim. The words (the way) and (the path) are materialistic objects used to indicate the abstract creeds.

(… إِنَّا هَدَيْنَاهُ ) (We guided him …): The Guidance here refers to the Guidance to the path that leads to the Paradise. It is not the Guidance of support, that is why it is followed by:

 (  إِمَّا شَاكِرًا وَإِمَّا كَفُورًا… ) (…be he grateful or be he ungrateful): Allah (swt) has bestowed His Blessings upon human beings by creating them out of nothing. Allah (swt) also prepared the means of provisions which are beyond our control and He (swt) prepared the universe to receive the human beings. These bestowments deserve our gratefulness and thankfulness. Out of Allah’s Mercy, He (swt) taught us to thank Him (swt) simply by two words: (All Praise to Allah). If Allah (swt) had not specified this formula of praising Him (swt), it would have been difficult for the humans to find a suitable formula to praise Allah (swt) for His Divine Perfection. No matter how the people are eloquent and have the ability of expression, they will not be able to find a formula which suits the Supremacy of The Giver, Who deserves to be praised for both His Giving and Withholding. His (swt) Existence and His Entity deserve praising. However, Allah (swt) connects between thankfulness and belief. Allah (swt) says:

 مَّا يَفْعَلُ اللَّهُ بِعَذَابِكُمْ إِن شَكَرْتُمْ وَآمَنتُمْ

(What would Allah do with your punishment if you are grateful and believe?) (An-Nisaa: 147)

So, what is the relation between thankfulness and belief? First, we should know what thankfulness means. Thankfulness means to praise the Giver by the one who receives His Blessings. As for the belief, it is to have certitude that Allah (swt) is One God. Thus, Allah (swt) shows people the Straight Path, as this verse clearly states. When human being believes that Allah (swt) guides them to the Stright Path, so they believe in His Messengers and Books, then this is an act of thanking Him (swt) for His Blessings. Thankfulness comes first then belief follows. Thankfulness is a general gratefulness and the belief is a special one. On the other hand, thankfulness relates to Blessings, while belief relates to the Entity that gives the Blessings. The humans should thank The Giver of Blessings. If the people had been aware of this fact, they would have thanked the Prophets and the Messengers for the Way of Guidance they conveyed from Allah (swt) because it leads them in managing the worldly affairs perfectly, most importantly, it guides them towards Paradise. Allah (swt) mentions the disbelief as the opposite of thankfulness, saying:

 وَإِذْ تَأَذَّنَ رَبُّكُمْ لَئِن شَكَرْتُمْ لَأَزِيدَنَّكُمْ ۖ وَلَئِن كَفَرْتُمْ إِنَّ عَذَابِي لَشَدِيدٌ

(And [remember] when your Lord proclaimed, ‘If you are grateful, I will surely increase you [in favor]; but if you deny, indeed, My punishment is severe) (Ibrahim: 7)

Allah (swt) wills to give the people the choice to thank Him (swt) or not and has not decreed them as grateful. Allah (swt) describes one of the thankful people; the Prophet Noah (PBUH), saying:

 ذُرِّيَّةَ مَنْ حَمَلْنَا مَعَ نُوحٍ ۚ إِنَّهُ كَانَ عَبْدًا شَكُورًا

(O descendants of those We carried [in the ship] with Noah. Indeed, he was a grateful servant.) (Al-Israa: 3)

A grateful servant, this expression indicates that he was thankful continuously and habitually, not just one time. As there are grateful people, there is, ungrateful ones, who are not just disbelievers, but they also lead others to disbelief and show ungratefulness.