त्रिविधा भवति श्रद्धा देहिनां सा स्वभावजा ।सात्त्विकी राजसी चैव तामसी चेति तां शृणु॥
tri-vidhā bhavati śraddhā dehināṃ sā svabhāva-jāsāttvikī rājasī caiva tāmasī ceti tāṃ śṛṇu
Word by word
The verse says
Faith in people comes in three kinds, and it grows out of what a person already is. Clear, driven, or dark. Listen.
In plain words
The answer begins by refusing the question slightly.
Arjuna asked which of the three they are. Krishna says: all three exist, in anybody, and what you get depends on what the person already is.
Svabhāva-jā - born of one's own nature. Your faith is not a decision you once made. It comes out of the material you are already made of.
Where you've felt this · in family life
Three sisters are brought up in the same house with the same prayers said at the same hour. One finds it steadying and keeps it all her life. One does it carefully and would be embarrassed to be caught missing it. One stopped at nineteen and has never gone back. Nobody taught them differently. The same thing went into three people and came out as three different things.
Going deeper
Faith growing out of what you already are is a less comfortable idea than faith being chosen, and it is also more accurate.
Svabhāva means your own becoming - the particular grain of a person, laid down by everything that has already happened to them. Faith grows in that soil. Which is why two people can hear the same teaching and one is changed and the other unmoved, with no dishonesty on either side.
This takes something away and gives something back. It takes away the right to feel superior about your convictions, since you did not pick them out of the air. It gives back a sane way of looking at other people, whose convictions arrived exactly the way yours did.
And it points at the only work that helps. You cannot talk yourself into a different faith. You can change the soil, slowly, by what you spend your days doing.