(Even) if you desire that they be guided, then verily, Allah does not guide those whom He allowed to stray, and they will have no helpers.
Allah told His Messenger that His eagerness to guide them will be of no benefit to them if Allah wills that they should be misguided, as He says;
وَمَن يُرِدِ اللَّهُ فِتْنَتَهُ فَلَن تَمْلِكَ لَهُ مِنَ اللَّهِ شَيْيا
And for whoever Allah wills to try with error, you can do nothing for him against Allah. (5;41)
Nuh said to his people;
وَلَا يَنفَعُكُمْ نُصْحِى إِنْ أَرَدْتُّ أَنْ أَنصَحَ لَكُمْ إِن كَانَ اللَّهُ يُرِيدُ أَن يُغْوِيَكُمْ
"And my advice will not profit you, even if I wish to give you good counsel, if Allah's will is to keep you astray." (11;34)
In this Ayah, Allah says;
إِن تَحْرِصْ عَلَى هُدَاهُمْ فَإِنَّ اللّهَ لَا يَهْدِي مَن يُضِلُّ
(Even) if you desire that they be guided, then verily, Allah does not guide those whom He allowed to stray,
As Allah says;
مَن يُضْلِلِ اللَّهُ فَلَ هَادِيَ لَهُ وَيَذَرُهُمْ فِى طُغْيَـنِهِمْ يَعْمَهُونَ
Whomsoever Allah allows to stray, then there is no guide for him; and He lets them wander blindly in their transgressions. (7;186)
إِنَّ الَّذِينَ حَقَّتْ عَلَيْهِمْ كَلِمَةُ رَبِّكَ لَا يُوْمِنُونَ
وَلَوْ جَأءَتْهُمْ كُلُّ ءايَةٍ حَتَّى يَرَوُاْ الْعَذَابَ الاٌّلِيمَ
Truly! Those deserving the Word (wrath) of your Lord will not believe, even if every sign should come to them - until they see the painful torment. (10;96-97)
فَإِنَّ اللّهَ
(then verily, Allah),
meaning, this is the way in which Allah does things. If He wills a thing, then it happens, and if He does not will a thing, then it does not happen.
For this reason Allah says;
لَا يَهْدِي مَن يُضِلُّ
(Allah does not guide those whom He allowed to stray),
meaning the one whom He has caused to go astray, so who can guide him apart from Allah No one.
وَمَا لَهُم مِّن نَّاصِرِينَ
And they will have no helpers.
means, they will have no one to save them from the punishment of Allah,
أَلَا لَهُ الْخَلْقُ وَالاٌّمْرُ تَبَارَكَ اللَّهُ رَبُّ الْعَـلَمِينَ
Surely, His is the creation and commandment. Blessed is Allah, the Lord of all that exists! (7;54)