QUL

If Allah had willed, He would have made all of Humanity one Nation

Allah says:

(And had Allah willed, He would have made you) meaning - O mankind,

((all) one nation,) This is like the Ayah:

(And had your Lord willed, all of those on earth would have believed together.) 10:99, meaning, He could have created harmony among them, and there would not be any differences, conflicts or hatred between them.

(And if your Lord had so willed, He would surely, have made mankind one Ummah nation or community, but they will not cease to disagree. Except him on whom your Lord has bestowed His mercy, and for that did He create them.) (11:118-119) Similarly, Allah says here:

(but He allows whom He wills to stray and He guides whom He wills. ) Then on the Day of Resurrection, He will ask them all about their deeds, and will reward or punish them even equal to a scalish thread in the long slit of a date stone or the size of a speck on the back of a date stone, or even a thin membrane of the date stone.

The Prohibition on taking an Oath for Purposes of Treachery

Then Allah warns His servant against taking oaths as means of deception, i.e., using them for treacherous purposes, lest a foot should slip after being firmly planted. This is an analogy describing one who was on the right path but then deviated and slipped from the path of guidance because of an unfulfilled oath that involved hindering people from the path of Allah. This is because if a disbeliever were to find that after having agreed to a covenant, then the believer betrayed him, then the believer will have hindered him from entering Islam. Thus Allah says:

(and you taste the evil of having hindered from the path of Allah, and you will suffer a terrible punishment.)

Do not break Oaths for the sake of Worldly Gain

Then Allah says:

(And do not use an oath by Allah for the purchase of little value.) meaning, do not neglect an oath sworn in the Name of Allah for the sake of this world and its attractions, for they are few, and even if the son of Adam were to gain this world and all that is in it, that which is with Allah is better for him, i.e., the reward of Allah is better for the one who puts his hope in Him, believes in Him, seeks Him and fulfills his oaths in the hope of that which Allah has promised. This is why Allah says:

(if you only knew. Whatever you have will be exhausted,) meaning, it will come to an end and will vanish, because it is only there for a certain, limited time.

(and what is with Allah will remain.) meaning, His reward for you in Paradise will remain, without interruption or end, because it is eternal and will never change nor disappear.

(And to those who are patient, We will certainly grant them their rewards in proportion to the best of what they used to do. ) Here the Lord swears, with the Lam of affirmation, that He will reward the patient for the best of their deeds, i.e., He will forgive them for their bad deeds.