Sacraments like baptism and the Eucharist, along with ideas of a dying-and-rising savior, were borrowed by early Christianity from surrounding Greco-Roman mystery religions such as the cult of Mithras, which had comparable initiation rites and communal meals long before Christianity emerged.

Why people find it persuasive

Mystery religions with initiation-by-water rites and sacred communal meals were widespread across the Roman world, and Christianity emerged and spread within that exact cultural milieu — it would be surprising if it developed its central rituals in total isolation from surrounding religious practice.

Important distinctions

• • General cultural resemblance (water rites, sacred meals exist in many religions) vs. demonstrated direct literary or historical borrowing.
• • Mithraic (and other mystery cult) evidence that postdates Christianity vs. evidence that clearly predates it.
• • Christian sacraments' actual Jewish liturgical roots (Passover, Jewish ritual washing/mikveh) vs. claimed pagan origins.
• • Superficial similarity in description vs. substantive similarity in theological meaning and function.

Christian response

• • The chronology is the single biggest problem for the parallel: most of our detailed evidence for Mithraism as practiced in the Roman Empire (the tauroctony imagery, the specific initiation grades, the sacred meal) dates from the late 1st century AD onward and flourishes mainly in the 2nd-4th centuries — after, not before, the New Testament's core claims and practices were already established; scholars like Edwin Yamauchi and Ronald Nash have documented that many 'parallels' popularized in the 19th and early 20th centuries (and still repeated online) rely on later sources being read back anachronistically as if they existed in fully-formed detail before Christianity, when the actual evidentiary base for pre-Christian Mithraism in the developed Roman form is remarkably thin.
• • Christian baptism has clear, well-documented, pre-Christian Jewish roots that require no pagan borrowing to explain: Jewish ritual washing (mikveh, tevilah) for purification, and specifically John the Baptist's baptism of repentance which the Gospels present as directly continuous with and prior to Christian baptism, situates the practice squarely within Second Temple Jewish religious life, the actual cultural matrix in which Jesus and his earliest followers operated, rather than requiring an appeal to distant Persian-derived mystery cults for explanation.
• • Similarly, the Lord's Supper is explicitly and directly rooted in the Jewish Passover meal (the Synoptic Gospels present the Last Supper as a Passover seder, and Paul explicitly calls Christ 'our Passover lamb' in 1 Corinthians 5:7), giving it an obvious, textually attested, historically proximate Jewish origin that requires no appeal to comparatively remote and later-attested Mithraic ritual meals to explain its existence.
• • The alleged 'dying and rising god' parallels (Osiris, Attis, Adonis, Mithras) have been shown by careful comparative-religion scholarship (notably Jonathan Z. Smith, himself not a Christian apologist but a leading historian of religion, in his critique of the 'dying-and-rising god' category) to be considerably less similar to the bodily, historically-claimed, one-time resurrection of Jesus than popular presentations suggest — many of these deities' 'deaths' involve seasonal disappearance-and-return myths tied to agricultural cycles with no claim to a single historical event or empty tomb, and some (like Mithras) are not clearly said to die and rise at all in the primary sources, making the entire comparative category itself considered questionable by mainstream religious studies scholarship.
• • Even where superficial resemblances exist (initiation rites, communal meals — practices found across an enormous range of unrelated human religious traditions because they naturally express religious ideas about washing/purity and shared identity/communion), the theological substance is quite different: Christian sacraments are tied to a specific, dateable, claimed historical event (the death and resurrection of a particular Jewish teacher under Pontius Pilate) with immediate soteriological and ethical content rooted in Jewish monotheism, not to seasonal fertility myths or secret philosophical illumination reserved for initiates, which is the actual defining feature of the mystery religions.

Christian counterresponse

This weaker claim about language and framing accommodating an audience is much more modest and plausible than the strong 'Christianity copied its central rites from pagan mystery religions' claim the objection usually intends, and it is unremarkable — all communicators use available cultural vocabulary to convey new content (much as contemporary preachers use modern metaphors), without that implying the content itself, particularly a doctrine as historically anchored and textually early as the resurrection and its associated sacraments rooted in Passover and Jewish purification rites, was actually derived from those pagan sources rather than being independently and demonstrably grounded in the Jewish and historical Jesus-tradition matrix.