Why did early Christians believe in the Real Presence? I'd imagine that if Christ directly taught the doctrine of the Real Presence, then one of the Gospels would have recorded it. So given that fact that it isn't directly taught, let us assume for the sake of argument that Christ did not explicitly teach this doctrine.

If this is the case, why did the early Christians seem to universally assume this doctrine to be true? Most notable example of this is St. Justin Martyr's First Apology. (I also say it seems to be "universally" assumed because nobody argued against the idea. This implies people thought of it as a typical Christian teaching.)

The typical proofs I hear from Catholic apologists are:


a certain reading of John 6
an insistence that the Words of Institution should be taken literally
a long proof of the mass as a propitiatory sacrifice (not merely a sacrifice of praise) thus implying the victim of this sacrifice must be Christ


But I cannot find any of these arguments in the early Christian writings. Further, it seems implausible that early Christians would even mount some of these arguments. i.e. it seems unlikely that St. Justin Martyr argued for the Eucharist as a propitiatory sacrifice considering that he hardly quotes the NT (he vaguely references the Gospels and Revelation). He displays little to no knowledge of the Apostolic letters.

If you know of any early Christians making one of these three arguments, let me know.

Now, the fourth argument Catholic apologists make is from the testimony of the early Christians! So this leads me to my question. On what basis did the early Christians get this idea of the Real Presence in the first place? More specifically, if Christ didn't teach the doctrine of the Real Presence explicitly, where did the apostles/early Christians supposedly get the idea?

Update: Some on another forum have simply asserted that the Apostles themselves verbally taught the early Christians the doctrine of the Real Presence. However, this just pushes the question one step further. Where did the Apostles get the idea from if Jesus did not explicitly teach it? Did the Apostles simply take the Words of Consecration literally at the Last Supper? Any theory is welcome (though citing a scholar who proposes a theory is best). This is indeed a very speculative question.

Update 2: Some users are trying to close this question on the basis that I “falsely” assume that Christians from the 2nd century onward generally believed in the Real Presence. I would hope people’s personal theological beliefs are not driving this movement, as it is indeed historically factual that Christians from the 2nd century onward believed Christ was truly present in the Eucharist. This is a theoretical question about the development of the belief in the Real Presence, not an apology for Catholicism or Protestantism. Please base your answers in history, not personal theological opinions.
J. N. D. Kelly (Early Christian Doctrines) contends that the early conception of the real presence was the result of a connection made between the OT sacrificial system and the eucharist.

Citing the Didache, Justin Martyr, and Irenaeus, he writes:


  Malachi's prediction (1, 10 f.) that the Lord would reject the Jewish sacrifices and instead would have 'a pure offering' made to Him by the Gentiles in every place was early seized upon by Christians as a prophecy of the eucharist. (196)


Others made the connection as well, like Clement and Ignatius.  Kelly argues that this was "natural":


  It was natural for early Christians to think of the eucharist as a sacrifice. The fulfilment of prophecy demanded a solemn Christian offering, and the rite itself was wrapped in the sacrificial atmosphere with which our Lord invested the Last Supper. The words of institution, 'Do this', must have been charged with sacrifical overtones for second-century ears; Justin at any rate understood them to mean, 'Offer this'. (196)


From here we need to identify what they considered the sacrifice to be.  In the case of Justin, the eucharist is clearly "much more than an act of purely spiritual recollection":


  Altogether it would seem that, while his language is not fully explicit, Justin is feeling his way to the conception of the eucharist as the offering of the Saviour's passion. (197)


Irenaeus's take is similar:


  The idea of the passion pervades this approach too, for Irenaeus identifies the gifts with Christ's body and blood and describes them, in language reminiscent of the Lord's words at the Last Supper, as 'the oblation of the new covenant'. (197)


Thus the eucharist is seen as a sort of continuation of the OT sacrificial system under the new covenant, and as such it makes sense that such sacrifices would need to be more than mere bread and wine.

Another potential factor, mentioned by Kelly in passing, is the early church's fight against Docetism, the idea that Christ did not have a real body.  He notes Ignatius's defense of the real presence in that context:


  Ignatius roundly declares that 'the eucharist is the flesh of our Saviour Jesus Christ, which suffered for our sins and which the Father in His goodness raised'. The bread is the flesh of Jesus, the cup His blood. Clearly he intends this realism to be taken strictly, for he makes it the basis of his argument against the Docetists' denial of the reality of Christ's body. (197)