The Surah derives its name from the sentence yuqatiluna fi sabil-i- hlsaff-an of verse 4; thereby implying that it is a Surah in which the word saff occurred.
It could not be known from any reliable tradition, but a study of its subject-matter shows that this Surah probably was sent down in the period closely following the Battle of Uhud, for by reading between the lines perceives a clear description of the conditions that prevailed in that period.
Its theme is to exhort the Muslims to adopt sincerity in Faith and to struggle with their lives in the cause of Allah. It is addressed to the Muslims with weak faith as well as those who had entered Islam with a false profession of the Faith and also those who were sincere in their profession. Some verses are addressed to the first two groups, some only to the hypocrites, and some only to the sincere Muslims. The style itself shows where one particular group has been addressed and where the other.
At the outset the believers have been warned to the effect; "Allah indeed hates those people who say one thing and do another, and He indeed loves. those who fight in the cause of the Truth, standing like a solid structure, against the enemies of Allah."'
In vv. 5-7 the people of the Holy Prophet's community have been warned that their attitude towards their Messenger and their Religion should not be like the attitude that the Israelites had adopted towards the Prophets Moses and Jesus (peace be upon them). In spite of acknowledging the Prophet Moses as a Messenger of God they continued to malign him as long as he lived, and in spite of witnessing clear signs from the Prophet Jesus they denied him without any hesitation. Consequently, the Israelites became perverse, incapable of benefiting from divine guidance. This is certainly not an enviable state which another nation should imitate.
Then, in vv. 8-9 a proclamation has been made with the challenge:"The Jews and the Christians, and the hypocrites, who are conspiring with them, may try however hard they may to extinguish this Light of Allah, it will shine forth and spread in the world in all its fullness, and the Religion brought by the true Messenger of Allah shall prevail over every other religion however hateful it may be to the pagans and polytheists.
In vv. 10-13, the believers have been told that the way to success both here and in the Hereafter is only one: that they should believe in Allah and His Messenger sincerely and should exert their utmost in Allah's Way with their selves and their wealth. As a reward for this they will earn immunity from Allah's punishment, forgiveness of their sins and the eternal Paradise in the Hereafter, and will be blessed with Allah's good pleasure, succor and victory in the world.
In conclusion, the believers have been exhorted to' the effect that just as the disciples of the Prophet Jesus had helped him in the cause of Allah, so should they also become "helpers of Allah", so that they too are blessed with the same kind of good pleasure and approval of Allah as had been the believers before them against the disbelievers.
Source: Sayyid Abul Ala Maududi - Tafhim al-Qur'an - The Meaning of the Quran
يَا أَيُّهَا الَّذِينَ آمَنُوا كُونُوا أَنصَارَ اللَّهِ كَمَا قَالَ عِيسَى ابْنُ مَرْيَمَ لِلْحَوَارِيِّينَ مَنْ أَنصَارِي إِلَى اللَّهِ ۖ قَالَ الْحَوَارِيُّونَ نَحْنُ أَنصَارُ اللَّهِ ۖ فَآمَنَت طَّائِفَةٌ مِّن بَنِي إِسْرَائِيلَ وَكَفَرَت طَّائِفَةٌ ۖ فَأَيَّدْنَا الَّذِينَ آمَنُوا عَلَىٰ عَدُوِّهِمْ فَأَصْبَحُوا ظَاهِرِينَ