The Quran

مَن كانَ يَظُنُّ أَن لَن يَنصُرَهُ اللَّهُ فِي الدُّنيا وَالآخِرَةِ فَليَمدُد بِسَبَبٍ إِلَى السَّماءِ ثُمَّ ليَقطَع فَليَنظُر هَل يُذهِبَنَّ كَيدُهُ ما يَغيظُ

Muhammad Asad

If anyone thinks that God will not succour him¹ in this world and in the life to come, let him reach out unto heaven by any [other] means and [thus try to] make headway:² and then let him see whether this scheme of his will indeed do away with the cause of his anguish.³

I.e., that God is not enough to succour him: obviously an allusion to the type of man who "worships God on the border-line of faith" (verse 11 above) and therefore doubts His power to guide men towards happiness in this world and in the hereafter. The assumption of the majority of the commentators that the personal pronoun "him" relates to the Prophet Muhammad is, to my mind, very far-fetched and certainly not warranted by the context.
The rendering of la-yaqta' as "let him [try to] make headway" is based on the accepted, tropical use of the verb qata'a (lit., "he cut") in the sense of "traversing a distance": and this is the interpretation of yaqta by Abu Muslim (as quoted by Razi). The expression "by any [other] means" (bi-sabab) relates to what has been said in verses 12-13 above.
Lit., "that which causes anger" or "exasperation", i.e., anguish at finding himself helpless and abandoned.