The first answer to the question

Who are those who believed

They are one of the three nations into which Allah classified the types of people, since the mission of our master Muhammad until the Day of Judgment. The classification of Allah came in the first verses of Surat Al-Baqarah, according to the actions of each nation, not according to their words, nor according to their classification of themselves, nor according to the name of their sects by which people know them.

The three nations are: those who believed, those who disbelieved, and those who deceive Allah and those who believed.

So Allah described the actions of those who believe as they establish prayer and spend charity from what Allah has provided them. They also believe in the unseen and in what was revealed to Muhammad and those before him, and they are certain of the Hereafter.

The evidence for this answer is the following clear verses from the beginning of Surat Al-Baqarah:

In the name of God, the most gracious, the most merciful

(Alif, Lam, Meem. (1) This is the Book about which there is no doubt, a guidance for those conscious of Allah (2) Who believe in the unseen, establish prayer, and spend from what We have provided them (3) And those who believe in what was revealed to you, [O Muhammad], and what was revealed before you, and of the Hereafter they are certain (4) Those are upon guidance from their Lord, and it is those who are the successful (5) Indeed, Those who disbelieve – it is all the same for them whether you warn them or do not warn them – they will not believe. (6) Allah has set a seal upon their hearts and upon their hearing, and over their vision is a covering, and for them is a great punishment. (7) And among the people are those who say, “We believe in Allah and the Last Day,” but they are not believers. They seek to deceive Allah and those who believe, but they deceive not except themselves, and they do not They feel (9) in their hearts a disease, so Allah has increased their disease, and for them is a painful punishment because they [habitually] used to lie (10))

The great Allah said the truth

Surat Al-Baqarah.