I think it is safe to say that the Reformed
do believe in the real presence of Christ in the Lord's Supper. The issue is not
whether he is really present, but
how he is really present. To put it more confessionally, how do we understand "sacramental union"? Geerhardus Vos explains very helpfully:
In what sense can one now say that Christ’s body and blood are present in the Lord’s Supper?
This then concerns the question of sacramental union (unio sacramentalis). We have now seen what the sign is; we have also seen what the thing signified is. The sign is bread and wine; the thing signified is Christ the Mediator in the unity of His death and His life. But what is the relationship between these two, sign and thing signified?
a) It is not a material union, or, expressed more precisely, a change, as if the bread and wine ceased being bread and wine and flesh and blood replaced them (against Rome).
b) It is not a local union, as if the body and blood of Christ were joined in space through local proximity or contact with the signs. We may not say: In this location where you see the signs, you must also consider the substance of flesh and blood present (against the Lutherans).
c) It is not a notional union that only exists in the minds of believers. With a symbol I always associate the thing signified with my conception; I represent it to myself. But that is a thing thought, something figurative. In the Lord’s Supper that is the starting point in our conscious faith, but sacramental union does not terminate in that (against Zwingli).
d) It is a union by relationship (σχετικῶς). The thing signified is associated with the sign for the believing participant. It does not come through the sign itself. The sign serves only to bring its meaning and certainty to our consciousness. Only at this point, and only to that extent, is the sign an instrument for the thing signified, expressed more precisely, for an awareness of the thing signified.
e) Not the existence of the thing signified, but certainly its spiritual efficacy on the soul of the communicant is tied to faith. One must pay attention to this point, and in this regard make a precise distinction. There is a fellowship with the living Mediator as He exists in a glorious human nature for the believing participant. But what if the participant does not believe? The thing signified does not thereby cease to exist. Christ remains the same Christ. However, He is only active by His Spirit in the heart of the believer to bestow fellowship with Himself. Now everything depends on what one understands by the presence of Christ. If this means that the signs, as commissioned by Christ, are offered by Christ, and that the Holy Spirit, as God omnipresent, is present, ready to apply the thing signified, then Christ is also present in the Lord’s Supper for the unbelieving participant. If, on the other hand, it means that by the Holy Spirit fellowship with the living Savior is established, then Christ is not present for the unbelieving participant. The word “presence” has more than one meaning. It can mean that something makes itself felt and causes its action to be experienced. It can also mean that something is present.
Faith is required in order to enjoy the blessing of the Lord’s Supper, in order, in the sense just described, to receive the body and blood of Christ present (as active power). Still, one may not say that faith makes Christ present. Faith is the conditio sine qua non [necessary condition], but it does not perform a miracle; it does not draw the human nature of Christ down from heaven.
—Geerhardus Vos, Ecclesiology, ed. Kim Batteau and Allan Janssen, trans. Richard B. Gaffin, vol. 5, Reformed Dogmatics (Bellingham, WA: Lexham Press, 2016), 237-239; italics original.