It is not like doing good...
It is not like doing good, being humble before God or purity of intention, which are counted as various grades of the faith. Piety, on the other hand, is a comprehensive virtue that runs through all the ranks of the true faith. It is for this reason that Allah has not reserved this adjective for any particular group of the believers.
The characteristics of piety, enumerated in these four verses, are five: Believing in the unseen, keeping up prayers, spending benevolently out of what Allah has given, believing in what Allah has revealed to His apostles, and being sure of the hereafter. The pious ones acquire these spiritual qualities by guidance from Allah, as Allah tells us in the next verse: “These are on a guidance from their Lord ”. They became pious and guarded themselves against evil because Allah had guided them to it.
When they got that quality, the Qur'an became guidance for them: “This Book . . . (is) a guidance to those who guard against evil”. It clearly shows that there are two guidances, one before they became pious, the other after it. The first guidance made them pious; and thereupon Allah raised their status by the guidance of His Book.
The contrast is thus made clear between the pious ones on one hand and the disbelievers and the hypocrites (who are admonished in the next fifteen verses) on the other. The later two groups are surrounded by two strayings and two blindnesses. Their first straying causes their unbelief and hypocrisy, and the second one (which comes after their unbelief and hypocrisy) confirms their first error and strengthens it.
Look at what Allah says about the disbelievers: Allah has set a seal upon their hearts and upon their hearing; and there is a covering over their eyes (2:7) . Sealing their hearts has been ascribed to Allah, but the covering over their eyes was put by the disbelievers themselves. Likewise, Allah says about the hypocrites: There is a disease in their hearts, so Allah added to their disease (2:10). The first disease is attributed to the hypocrites themselves, and the second one to Allah.
The same reality has been explained in many verses. For example: He causes many to err by it and many He leads aright by it! But He does not cause to err by it (any) except the transgressors (2:26); . . . but when they turned aside, Allah made their hearts turn aside (61:5). In short, the pious ones are surrounded by two guidances, as the disbelievers and hypocrites fall between two errors.