(only for the Hour that it shall come upon them suddenly while they perceive not) means, for it is real and will inevitably come to pass, and these negligent people are unprepared for it. When it comes, it will catch them unawares, and on that Day they will feel the utmost regret when regret will not benefit them in the slightest and will not afford them any protection.
(Friends on that Day will be foes one to another except those who have Taqwa.) means, every friendship that exists for a purpose other than for the sake of Allah will turn to enmity on the Day of Resurrection, except for that which is for the sake of Allah, which will last forever. This is like the statement of Ibrahim, peace be upon him, to his people:
The only real friendship is that which is in the sake of Allah
الْأَخِلَّاءُ يَوْمَئِذٍ بَعْضُهُمْ لِبَعْضٍ عَدُوٌّ إِلَّا الْمُتَّقِينَ (Friends, on that day, will become enemies to one another, except the God-fearing 43:67). This verse tells us plainly that the friendly relations in which we take so much pride in this world, and for which we don't even care about halal (permitted) and Haram (unlawful), will be of no use on the Day of judgment, rather they will turn into enmity. In this connection, Hafiz Ibn Kathir has reproduced a discourse of Sayyidna ` Ali ؓ narrated by ` Abdur Razzaq in his Musannaf and by Ibn Abi Hatim regarding two Muslim friends and two Non-Muslim friends. One of the Muslim friends died and when he was given the glad tidings of Paradise, he recollected his friend and supplicated in his favour: "0 Allah! That friend of mine used to advise me to obey You and to obey the Holy Prophet ﷺ ، he used to direct me towards virtues and to prevent me from vices, and kept on reminding me that one day I would have to meet You. So 0 Allah! Do not misguide him after me, so that he may also see the scenes of Paradise as You have shown to me, and be pleased with him, as you have been with me." In reply, it will be said to him, "Do not worry, if you know what reward I have stored for your friend, you will weep less and laugh more." Later, when the friend will also have died, both the souls will get together and Allah Ta` ala will direct them that each one of them should praise the other. Thereupon, each one of them will say about the other that he is the best brother, best companion and the best friend.
Man is not free. At all events, he has to humbly accept reality. If he does not defer to the reasoning of the preacher of Truth, he shall have to submit to godly power. But godly power comes into play only at the time of the Final Judgement. Therefore, doing obeisance only at that time and not before will be of no avail to anybody. In this world, when a man adopts any course which is opposed to the Truth, he finds many friends to support him. Man becomes bolder and bolder on the strength of such friendship. But all these friends will abandon him on the Day of Judgement. On that Day, only that friendship will last which has been established on the basis of the fear of God. A life which is dedicated to the Truth is fraught with dangers. But in the Hereafter, it will be permanently relieved of every type of fear or trouble. Only those who firmly believe in this godly assurance will remain steadfastly on the path of Truth in this world. In the life Hereafter, God will bestow on them much more than they lost in this world for His sake.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.
