If you have received Christ and are resting upon him alone for salvation, as he is offered to you in the gospel, if you are a baptized and professing communicant member in good standing in a church that professes the gospel of God's grace in Jesus Christ ... ["The Directory for the Public Worship of God," III.C.3. In: The Committee on Christian Education of the Orthodox Presbyterian Church, The Book of Church Order of the Orthodox Presbyterian Church (2011 Edition) (Willow Grove, PA: The Committee on Christian Education of the Orthodox Presbyterian Church, 2011)]
According to confessional Reformed practice and piety, who should be partaking of the Lord's Supper? If we take seriously the commands of 1 Corinthians 11:27-29, then we must say that the Lord's Supper is not open to anyone and everyone. Those who partake of it illegitimately eat and drink damnation upon themselves. I shudder when I see parents in certain (non-Reformed) churches break pieces of the consecrated bread and give it to their children. How can they hate their children so much to invite judgment upon them? But I digress.
If the Lord's Supper is not to be open to anyone and everyone, and most certainly it is to be taken in a spirit of reverence (instead of the irreverence of the Corinthians which had gotten them killed in judgment), then the Church ought to guard the Supper. The guarding of the Supper is not to protect the elements, for we do not believe they are transformed into the divine substance in any way. Rather, from the passage in 1 Corinthians, the practice of "guarding the Supper" is meant to protect those who partake of it. Guarding of the Supper is meant to protect the irreverent from eating and drinking further judgment upon themselves. As a sacrament given to the Church, it is the Church that administers it to God's people. Therefore, as a church-instituted ritual, other qualifications soon come to bear to ensure that the Lord's Supper is to be properly administered.
In the conflict that is the Reformation, the Reformers inquired into the nature of the true church. The signs of the true church, according to the Reformed tradition, are: the right preaching of the Word of God, the right administration of the Lord's Supper, and the right administration of church discipline. In order for the Lord's Supper to be properly administered, it must be administered in both elements (Bread and wine), by a minister rightly ordained, in a true church that preaches the Gospel, to church members who are not under legitimate church discipline. The phrase in Reformed circles for legitimate partakers of the Lord's Supper therefore can be framed to be for "members of good standing, in a church of like faith." It is of course agreed that only Christians can partake of the Supper. But besides that, the qualifications for someone to partake of the Supper is encapsulated in that phrase, which will be unpacked as follows.
The first qualification for partaking of the Supper is that someone is a church member. Since the Supper is an ecclesial ritual, a rite given to the Church, only those who are members of churches can partake. Since we are saved into the church, it stands to reason that those without membership in a church are not affiliated with the institution of salvation and thus should not partake of the Supper.
The second qualification is that the one partaking is a communicant member. This is obvious since the Supper is to be done "in remembrance of me." Non-communicant covenant children have not yet come to a profession of faith and thus cannot fulfil what is commanded of believers to do.
The third qualification is that the member must be a "member in good standing." This means that the member is not being placed under legitimate church discipline for sins committed in his life. Grievous unrepentant sin is a scourge in the church, and must be dealt with just as Paul did in 1 Corinthians 5:5. The term "legitimate" is stressed since the false church always persecutes the true church, just as Martin Luther and the Reformers were excommunicated by the apostate Late Medieval Catholic Church.
Fourthly, the one who partakes must be in a member in "a church of like faith." What this means is that it is recognized that his church is a true church "of like faith." The Reformers struggled against both the emerging Roman Catholicism and the Anabaptists. If the church is a false church, then someone being a member therein means nothing whatsoever since that is not a church. Progressing further in history, we have the Arminian controversy in early 17th century Dutch Reformed Christianity, where the Classical Arminians were kicked out of the church for false teaching. Some of the expelled ministers in time form their own Arminian churches. Since these ministers are under church discipline themselves, their churches are not biblical and neither can membership in them be considered true church membership in true churches.
With the emergence of Evangelicalism in the late 18th century, the issue of what constitute a true church "of like faith" becomes more difficult to discern. Evangelicalism to a large extent believes in the true Gospel, so it seems that members in their churches can be considered members of churches "of like faith." On the other hand, particularly with New Evangelicalism, heretics of all sorts abound within, extending even to entire churches. So how does one discern whether a membership in a church is a membership of a "church of like faith"?
Concerning this, much discernment and wisdom is required. In some sense, it can be said that Evangelical churches meet the criteria for being "churches of like faith." Yet, in another sense, they do not since many of them tolerate heretics and heresies in their midst. Thus, the "line" if you will is not so clear cut, running somewhere between Evangelicalism broadly and confessional Reformed churches only, narrowly. Generally speaking, members in Reformed churches can be considered "of like faith" while those in Evangelicalism should be evaluated on a case by case basis.
Who are those therefore who can partake of the Lord's Supper? They are to be believers, members in good standing, in a church of like faith. Failing to meet either of these criteria means that one should not partake of the Supper, even if one truly believes in Christ for salvation, so that the Supper is not defiled by the stain of someone taking it improperly, resulting in judgment upon the one partaking.