"Those who disbelieved": Those who refused to accept the teachings and guidance presented 'by Muhammad (upon whom be Allah's peace and blessings).
The verb sadd (from which saddu of the original is derived) is used both as a transitive and as an intransitive verb in Arabic. Therefore, the sentence would mean "they themselves refrained from adopting Allah's Way" as well as they prevented others from adopting this way."
There are several ways of preventing others from adopting Allah's Way:
(1) That one should forcibly prevent another from believing;
(2) that one should so persecute the believers that it should become difficult for them to remain steadfast on the Faith and for others to embrace the Faith in view of the dreadful conditions;
(3) that one should mislead the people against the Religion and its followers in different ways and create doubts in the hearts so that the people may become suspicious of the Religion itself. Besides, every disbeliever prevents others from the way of Allah in the sense that he brings up his children on the way of disbelief, and then it becomes difficult for his future generation to give up the ancestral faith and accept Islam. Likewise, every unbelieving society is an obstruction to the Way of Allah, for it creates obstacles in the way of the progress of the Faith by its system of education and training, its government and its customs and prejudices.
The words adalla a 'mala-hum (led their deeds astray, let them go to waste) are very comprehensive in meaning. Their one meaning is: Allah deprived them of the grace that their efforts and their labors should be spent in the right way: now, whatever they do, they will do that for wrong aims and objects in wrong ways: all their endeavors will be directed to the way of error and deviation - instead of guidance. Their second meaning is: The works that they have been performing as pious deeds, e.g. guardianship of the Ka'bah, entertaining the guests, treating the kindred well, and such other works, which are counted among religious services and noble actions morally, were rendered fruitless by Allah; they will get no reward for them, for when they refuse to accept Tauhid and to adopt the way of Allah's service and worship and stop others also from adopting Islam none of their works can be acceptable to Allah. The third meaning is Allah rendered fruitless an$ vain the efforts they were making to obstruct the way of the Truth and to keep their creed of disbelief alive in Arabia as against the Prophet Muhammad (upon whom be Allah's peace and blessings): all their designs and endeavors now are aimless: they will never attain their goals by these designs.
Although after saying alladhina amanu (those who believed) there was no need to say wa amanu bi-ma nuzzila 'ala Muhammad-in (and accepted that which has been sent down to Muhammad), for belief by itself implies believing in the Holy Prophet Muhammad and in the teachings sent down to him, yet making a separate mention of it is particularly meant to assert this: After the appointment of Muhammad (upon whom be Allah's peace and blessings) to Prophethood, a person's belief in Allah and the Hereafter and the former Messengers and the Books cannot be beneficial until he also believes in him and the teachings brought by him. This explanation was necessary because after the emigration, the Muslims had also to deal with those people, who recognized and accepted all the requirements of the Faith but refused to acknowledge and accept the Prophethood of the Holy Prophet Muhammad (upon whom be Allah's peace and blessings).
This has two meanings:
(1) That Allah wiped off from their record all those sins which they had happened to commit in the pre-Islamic days of ignorance: now, they will not at all be held accountable for them; and
(2) that Allah removed from them the evils of creed, thought, morals and action in which they were involved. Now their minds were changed; their creed and ideas were changed; now there was faith in their hearts instead of ignorance and righteous acts instead of the immoral evil.
This also has two meanings:
(1) That Allah changed their previous condition and put them on the right path and improved and bettered their lives for them; and
(2) that Allah has taken them out of the condition of weakness and helplessness and oppression in which they were placed till then; now He has created for them such conditions in which instead of being persecuted they will defend themselves against the oppressors; instead of living as subjects, they will now live and order their lives as free people, and will have the upper hand instead of being subdued and suppressed.
That is, "He makes both the parties aware of their positions. One party insists on following falsehood; therefore, Allah has brought all its endeavors to naught: and the other party has adopted obedience of the Truth; therefore, Allah has purified it of its evils and corrected its condition."