The original sentence would be La foi est le mortier.le mortier cimente nos croyances We want to combine these 2 sentences to avoid repetition. The repeated word is the subject in the second sentence therefore we use "qui" to replace it. If it was an objectwe would have used "que" Ex: j'aime mes chaussures. Mon père m'a acheté des chaussures. The repeated word here is an object We combine them and we get J'aime mes chaussures que mon père m'a acheté I hope I answered your question

Qui here indicate a relative, it introduces a second sentence. As 나지하 it would avoid repetition and here would have a function. Here le mortier is subject of cimente. As a relative pronoun qui would indicate subject. Is it would be a complement (le ciment que j'ai acheté, for example), we would use que. Hope you will understand. ^^