Allah said;

ثُمَّ كَانَ مِنَ الَّذِينَ آمَنُوا ...

Then he became one of those who believed,

meaning, then, along with these beautiful and pure characteristics, he was a believer in his heart, seeking the reward of that from Allah.

This is as Allah says,

وَمَنْ أَرَادَ الاٌّخِرَةَ وَسَعَى لَهَا سَعْيَهَا وَهُوَ مُؤْمِنٌ فَأُولَـئِكَ كَانَ سَعْيُهُم مَّشْكُورًا

And whoever desires the Hereafter and strives for it, with the necessary effort due for it while he is believer, then such are the ones whose striving shall be appreciated. (17:19)

Allah also says,

مَنْ عَمِلَ صَـلِحاً مِّن ذَكَرٍ أَوْ أُنْثَى وَهُوَ مُؤْمِنٌ

Whoever works righteousness -- whether male or female -- while being a true believer.... (16:97)

Allah says,

... وَتَوَاصَوْا بِالصَّبْرِ وَتَوَاصَوْا بِالْمَرْحَمَة ِ ﴿١٧﴾

and recommended one another to patience, and recommended one another to compassion.

meaning, he was from the believers who worked righteous deeds, and advised each other to be patient with the harms of the people, and to be merciful with them.

This is similar to what has been related in the noble Hadith,

الرَّاحِمُونَ يَرْحَمُهُمُ الرَّحْمنُ،

ارْحَمُوا مَنْ فِي الْأَرْضِ يَرْحَمْكُمْ مَنْ فِي السَّمَاء

The merciful people will be treated with mercy by the Most Merciful (Allah).

Be merciful to those who are on the earth and He Who is above the heavens will be merciful to you.

In another Hadith he said,

لَا يَرْحَمُ اللهُ مَنْ لَا يَرْحَمِ النَّاس

Allah will not be merciful with whoever is not merciful with the people.

Abu Dawud recorded from Abdullah bin Amr that he narrated (from the Prophet),

مَنْ لَمْ يَرْحَمْ صَغِيرَنَا وَيَعْرِفْ حَقَّ كَبِيرِنَا فَلَيْسَ مِنَّا

Whoever does not show mercy to our children, nor does he recognize the right of our elders, then he is not of us.