Why have disagreements abounded in religion concerning matters such as congregational prayer in the mosque and the length of the beard, hindering the enjoining of good and forbidding of evil? And why did Allah not reveal definitive texts on these matters, leaving room for disagreement which the questioner sees not as a mercy but rather as leading to factions?

It is incorrect to say that "everything in religion is a matter of disagreement." The fundamental and prevalent aspect of religious issues is agreement. Matters of consensus far outnumber matters of disagreement, potentially reaching tens of thousands, whereas issues of disagreement do not exceed a thousand. Among these, some are tolerated, while others involve clear error. Disagreements occur in the jurisprudential branches due to previously stated reasons. A Muslim should engage in enjoining good and forbidding evil in matters that are agreed upon, for there is great engagement and immense virtue in it.