You are reading a tafsir for the group of verses 15:10 to 15:15

A Lexical Note of Explanation

The word: اَلشِيَعِ in the first verse (10) is the plural form of: شِیعہ (shah) which means the follower or helper of a person. Then, it is also used to denote a group which agrees upon particular beliefs and theoretical assumptions. So, the sense of the statement is that Allah Ta’ ala has sent messengers among every group or sect. Here, by using the word: فِی in فِي شِيَعِ الْأَوَّلِينَ ( in place of اِلٰی : ila: to (translated as 'among the groups of earlier peoples' ), the hint given is that the messenger for every group was sent from among that particular group so that people would find it easy to trust him, and that he too, by being aware of their taste and temperament, could make appropriate plans to work for their reform.