Shaykh Ṣāliḥ ibn Fawzān al-Fawzān:

As for the people of Sunnah, they were steadfast in practicing the Sunnah, the Book of Allah and the Sunnah. They rejected these deviant books and deviant beliefs. They totally rejected them and adhered to the book of Allah, the Sunnah of Allah’s Messenger and the creed of the righteous Salaf.

It is from the wisdom of Allah that He puts the people of Emān through afflictions by way of the people of falsehood so that the patient individuals who strive and are steadfast in their practice of the religion become distinct from those who in them is hypocrisy or doubts. Consequently, this person becomes distinct from that one.
Were it not for tribulations, the people of truth would not be distinct from the people of falsehood. Allah decrees these tribulations for a tremendous wisdom.

If the people were given free rein, the truth would become obscured by falsehood and a person would think that all the people are in accordance to the truth. But when the tribulations come the people are separated into two groups. The people of truth who are steadfast, and the people of falsehood who deviate and misguide others. By way of this, Allah filters out the rotten from the righteous and makes the rotten topple each other. This is the crux of afflictions, just as you witness nowadays.

When the tribulation came from the disbelievers against Islam and the people of Islam, individuals among the Muslims emerged in support of the disbelievers and propagated their call. They emerged from among the Muslims and the descendants of Muslims, and from among those ascribed to knowledge. Those who aligned themselves with the disbelievers emerged and supported them. They favoured their ideals and ideologies and promoted them.
This is the Sunnah of Allah in His creation. He puts his worshippers through afflictions to make the truthful believer distinct from the hypocrite and those who have a spiritual sickness in their heart. This distinction becomes apparent.

Do you think that you will be left alone, and Allah has not yet made it distinct who the strivers among you are, and those who are patient? [Q 3:142] and [Q 9:16].

“Allah will not leave the believers as you are until he filters out the rotten from the righteous. Allah will not disclose the secrets of the unseen to you” [Q 3:179]