Why did early Christians believe in the Real Presence?

To find out if the early Christians universally assumed the Catholic doctrine of the doctrine of the "Real Presence" to be true then one should examine the only wholly inspired and substantive record of what the early Christians believed, this being the New Testament, and in particular, Acts thru Revelation, which reveals how they understood the gospels.

In which,

1. Rather than being manifest asc being the "the source and summit of the Christian life," (CCC 1324) which is "the cause of that communion in the divine life," (CCC 1325) and the work of our redemption is carried out;" (CCC 1364) the Lord's supper (LS) is only manifestly described in one epistle, that being 1 Corinthians 10,11, besides the reference to the "feast of charity" in Jude 1:12. If the mention of breaking of break together in Acts is that of the LS, then it is that the disciples who from from "house to house, did eat their meat with gladness and singleness of heart."(Acts 2:46)

2. Rather than being "the same sacrifice with that of the cross...a sacrifice of propitiation, by which God is appeased and rendered propitious;” (The Catechism of the Council of Trent) it is nowhere described as a sacrifice for sins, but a communal meals which shows/proclaims the Lord's death for the church by unselfishly sharing food in a actual meal, thus showing unity with the object of that dedicatory feast, and each other, as did pagans in their religious feasts. See here.

3. Rather than only a duly ordained Catholic priest being able to conduct the LS, with the primary active duty priest being "most of all to offer the Eucharistic Sacrifice," (Pastoral Reflections on the Holy Sacrifice of the Mass, Cardinal John J. O'Conner) nowhere do we see this restriction, nor even Catholic priests in the life of the NT church, this being a separate class of sacerdotal believers for which the distinctive Greek word for such (hiereus)is translated as priests though it is actually a etymological corruption of the Greek "presbyterous" = senior/elder) See here.

4. Rather than consuming the elements in the LS being a primary means of spiritual nourishment that "gives life to the believer""the irreplaceable food for the journey of the pilgrim church on earth." (USCCP: "Norms for the Distribution and Reception of Holy Communion," paragraphs. 4,14) "the food that makes us live for ever in Jesus Christ," (CCC 1405) so that after their first sacramental confession children are to be "nourished by this divine food as soon as possible;" (Can. 914)

Nowhere is consuming the bread and wine described as providing this irreplaceable spiritual nourishment. Instead, spiritual life is obtained by hearing the gospel and truly believing it. (Acts 2:38; 10:43-47; 15:7-9; Eph. 1:13) And which provides spiritual nourishment by drinking "the sincere milk of the word," (1 Pt. 2:2) and ingesting its "meat," (1Co. 3:2; Heb. 5:12,13) being "nourished" (1Tim. 4:6) and built up by the word, (Acts 20:32) and with feeding the flock thereby being the primary active function of pastors, (Acts 20:32) besides prayer. (Acts 6:4)

In addition, the NT church certainly did not subscribe to the claimed but actually metaphysical Eucharistic theology of the Roman Catholicism, that of the "Real Presence" (though that term was apparently originally Anglican), that at the words of consecration by a validly ordained priest, the valid bread and wine, entirely in each one and in each of every particle (down to the most minute one), no longer actually exist (the bread and wine have ceased to exist after the consecration" - Pope John Paul II, Ecclesia de Eucharistia, 2003; cf. Summa Theologiae > Third Part > Question 75), but - regardless of any observable qualities or scientific tests to the contrary -have become the "true Body of Christ and his true Blood," (CCC 1376; 1381) having been "substantially changed into the true and proper and lifegiving flesh and blood of Jesus Christ our Lord," being corporeally present whole and entire in His physical "reality;" (Mysterium Fidei, Encyclical of Pope Paul VI, 1965) "the very body which he gave up for us on the cross, the very blood which he "poured out for many for the forgiveness of sins,"(CCC 1365) with His human body and human soul, with His bodily organs and limbs and with His human mind, will and feelings. (John A. Hardon, S.J., Part I: Eucharistic Doctrine on the Real Presence)

But since her priests do not effect the actual manifest physical changes which an actual purely literal reading of the “words of consecration” at the last supper would teach, then they must engage in an extensive metaphysical explanation to justify it. For this Real Presence is not as the manifestly incarnated Christ of Scripture, whose manifest physically is so much stressed in Scripture as being opposed to a docetist or gnostic-type Christ who appears to be something he is not.

For "the Most Holy Eucharist not only looks like something it isn't (that is, bread and wine), but also tastes, smells, feels, and in all ways appears to be what it isn't." (The Holy Eucharist BY Bernard Mulcahy, O.P., p. 22)

For while bread and wine are said to have ceased to exist and Christ Himself is instead what the partakers ingest, this is "not as "sensible, visible, tangible, or extended, although it is such in heaven," but is under a "new mode of being," under the mere appearance of this non-existent bread and wine.

But only until manifest decay begins:

“The Eucharistic presence of Christ begins at the moment of the consecration and endures as long as the Eucharistic species subsist.” (CCC 1377). At which point Aquinas argued that the substance of the bread and wine (which no longer exists) cannot return, despite appearance. (Summa Theologiae, Question 77)

To inhibit this decay, Catholic instruction requires that the bread used in the Eucharistic Sacrifice must be unleavened, while hosts that are completely gluten-free are invalid matter, which thus causes problems for many Catholics with Celiac disease due to the adverse effects to the non-existent gluten.

Therefore the answer to the question to as whether the early Christians universally assumed the Catholic doctrine of the doctrine of the "Real Presence" to be true is that this is simply not what is evidenced in the light of the only wholly inspired authoritative source on what they believed. Which was neither a purely literal understanding of the "words of consecration" much less that which is justified by somewhat using Aristotelian substance theory, while the metaphorical understanding alone easily conflates with Scripture overall. See here for an extensive examination of this issue, by the grace of God, with Eucharistic theology being just one of many Catholic distinctives not seen in the inspired record of what the NT church believed.

Besides the answers given, the whole issue of taking Communion "in an unworthy manner" is another reason that the ancient Christians believed in presence of Christ in the Eucharist. People don't usually get sick and die from things that are just symbols and token acts.

1 Corinthians 11

27 So then, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. 28 Everyone ought to examine themselves before they eat of the bread and drink from the cup. 29 For those who eat and drink without discerning the body of Christ eat and drink judgment on themselves. 30 That is why many among you are weak and sick, and a number of you have fallen asleep. 31 But if we were more discerning with regard to ourselves, we would not come under such judgment. 32 Nevertheless, when we are judged in this way by the Lord, we are being disciplined so that we will not be finally condemned with the world.

"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?"

They got the idea from the phenomena of the Holy Spirit transforming them. Their Spiritual transformation of being made new unto Sanctification conjured new, fuller meaning onto the holy ritual, and Christ's body and blood came to represent the Saints' bright, shining newness in Christ.

There is a real inwardly presence when one is born again. We live in a time where people are converted but not born of the Spirit. But being "baptized by the Spirit" was a supernatural phenomenon of receiving Christ's Spirit—body & blood. The Holy Spirit's role: (1) converts; (2) enlightens; (3) sanctifies. The Holy Spirit converts, but as the wind and waters works us toward the light, there's much more for the heart to receive to be made manifest (John 3).

Transferring the real habitation of the Holy Spirit onto the ritual that reminds us of the fact that the Anointed may anoint us, therefore, may blind men to the gift of Christ's sacrifice, which allows his character and light to be transferred to the repentant (Acts 2:38), thus washed in His blood. His sacrifice allowed for the anointing of gentiles and Jews alike. That is the REAL presence. And just as the Gnostics spoke of the Eucharist as in tongues—pointing toward the transmutative renewal unto an antichrist Spirit (having the likeness of but rejecting the power thereof)—so did the apostles partake in Spiritual food and drink unto His glory.

“Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea; And were all baptized unto Moses in the cloud and in the sea; And did all eat the same spiritual meat; And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ.” 1 Corinthians‬ ‭10:1-4‬ ‭KJV‬‬

"Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost." Acts 2:38

"But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you. Therefore, brethren, we are debtors, not to the flesh, to live after the flesh." Romans 8:11, 12

The basic assumption, "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.", should be changed to "Since scripture depicts Jesus directly teaching at the Last Supper that he was giving himself to us and not merely passing along a symbol, supported by his plain teaching in John chapter 6 that his body is real food and his blood is real drink, we will examine whether the Early Church confirmed or rejected what Jesus directly taught."

This revised assumption is supported easily by the evidence of the Early Church specifically that the Eucharistic is the Real Presence. Albeit not a strong argument, there is no early Church writing or early practice to refute this direct teaching by Jesus.

The timing of the NT books authorship (additionally the late compilation of the NT within the Bible centuries later) give weight to the premise that Jesus taught his flesh was real food and his blood was real drink (John Chapter 6).

Paul was the first NT author by decades. He writes in 1Corthians of the tradition of the Eucharist as it already exists in the Early Church including the words of institution "This is my body" and "This is my blood". He writes that he received these instructions from the Lord.

How he received this information from the Lord ie: the words of institution (as Catholics refer to the lines pronounced at the consecration) is commonly thought to have come from the Apostles' teaching. This would indicate that the Apostles took seriously that the Lord was giving himself in the Eucharist. No early writing change this "formula" using overtly words meaning symbol or write casually about his being just bread or wine. Rather the formula from all three synoptic gospels and Paul himself are kept the same. The importance of this cannot be underestimated when the span of time in which the Gospels were written is decades after Paul wrote. Yet the formula across time and distance and culture remains in tact. This cannot be said of other aspects of ther earely Church, such as two reditions of the Our Father (Luke and Matthew) or variations in miracle stories.

In any case, Paul received an ongoing belief before the gospels were even written in which Paul warns not receive the Eucharist unworthily. This speaks plainly that Paul considered the Eucharist to be sacred, of God and not merely symbolic bread.

Finally, Paul and most early Christians were practicing Jews. Per Paul's writing and the Acts of the Apostles we read that Jewish-Christians continued pursuing Judaism with their new belief in Jesus Christ. The notion that a practicing Jew would change their most important Jewish celebration of Passover in order to receive a mere symbol of bread is highly unlikely. The magnitude of the change would require something more important being introduced. Symbolic bread would not compel this incredibly important feast to be changed. Jesus as the Lamb of God was the difference. Just in the old Passover the Lamb had to be eaten, so to in the new Passover the Lamb of God was to be consumed. It was no mere symbol to them who understood what the Passover meal was about.