English sincerity, Quranic ikhlas and Confucian xin are commonly given as translations of one another, and each names something a reader would recognize as a virtue of truthfulness. They are not, on inspection, the same virtue, and treating them as interchangeable loses exactly what each tradition thought was worth naming separately.
Aristotle's sincerity, as the article beside this one sets out, concerns self-report: whether a person represents their own qualities and achievements accurately, neither boasting nor understating. Ikhlas, in the vocabulary of Islamic ethics, concerns intention rather than self-report. It is purity from admixture, the state of directing worship and good deeds toward God alone without any secondary motive of human praise mixed in, a fault the tradition names riya and treats as a distinct and serious spiritual danger in its own right. A person could satisfy Aristotle's sincerity, giving an accurate account of their own good deeds, while entirely failing ikhlas, if the deeds themselves were performed for the sake of being seen doing them.
Xin, one of the Confucian constant virtues, concerns neither self-report nor intention but the keeping of one's word. The character combines a person and speech, and the virtue it names is reliability, doing what one has said one will do, the trustworthiness that makes an oath or a promise mean something. It is closer to the Aristotelian virtue of a different name, keeping faith, than to truthfulness about oneself.
The three sit near each other in ordinary translation because English has one common word, sincerity, standing in for self-report, purity of intention and reliability of word alike. The traditions that produced ikhlas and xin drew the boundaries differently, and each is described here in its own terms rather than folded into a single cross-tradition virtue.