True worship can exist without congregational liturgy. Worship is necessary for salvation; liturgy is not. Those who reduce their religious experience to the external forms of worship will not be saved.
Now, Catholic theology welcomes all of these claims. Christian worship can be as simple as a prayer, a glance, or a breath, but can also be as extravagant as a liturgy or procession. In either case, however, true worship is realized "in spirit and in truth," in one's own heart, by faith. We can all agree on this point.
However, it seems another sentence in the article's final paragraph is being used to suggest that liturgy is antithetical to true worship: "Participation in liturgical forms and ceremonies is not worship." Taken in isolation, this is a much more troubling proposition.
To make "liturgical forms and ceremonies" antithetical to "worship" is, as "Lady Dragon" noted, radically out of keeping with the biblical tradition. To defend this contrast, one would have to claim that the only form of worship God ever prescribed in scripture (liturgy/ritual, as in the Psalms, Leviticus-Deuteronomy, and even as far back as Genesis, etc.)--the form of worship Adventists themselves believe is now being performed in the Heavenly Sanctuary (!) (cf. mentions of incense, altars, ritual clothing, and extravagant adornments in Rev. 2; 4-5; 8, etc.)--is not worship at all. This is a chilling conclusion. The tendency to make worship and liturgy an "either/or" issue is, simply put, an unbiblical approach to the question of worship. In Sacred Scripture, worship and liturgy are always "both/and."
Lest we forget, even our humblest prayer on earth is, ultimately, part of that vast and superlative liturgy described in Revelation. Every prayer we whisper is being presented as incense by the priestly ministries of angels and other heavenly beings, holding golden vessels (Rev. 5:8; 8:2-3). The fact is, all Christian worship is, in the final analysis, liturgical. All Christian worship is channeled into, and configured within, the celestial liturgy. Thus, we can never speak of worship and liturgy as antithetical concepts--especially not in the New Testament. In the New Testament, the liturgy of heaven encompasses all worship that exists in all the universe (something that might not be said of Old Testament liturgics, restricted by earthly limitations).
Now, does this mean that Christian worship here below must, or must not, be ordered and formalistic? The fact is, the New Testament does not tell us, and for a very obvious reason. No epistle of the New Testament was intended to establish a church, including, most especially, its rhythms of worship. Rather, the apostles established the rhythms of local churches through their oral preaching, instruction, and example, when gathering followers and ordaining local leaders. The basic shape of Christian worship would have been delineated in these initial missionary labors, and not in later correspondence with these communities. As is easily expected, the epistles only deal only with particular controversies (e.g., speaking in tongues and interpretation [1 Cor 14], seating the wealthy and poor [1 Cor 11], etc.) that presuppose an existing, regular life of worship.
Let us assume for a moment that whatever is not contained in scripture is not of any binding force (a common Protestant assumption). In that case, no one can insist upon, or exclude, the practice of ordered, formal, liturgical worship in Christianity. Scripture itself gives no one license to claim that prayers must be spontaneous or aliturgical. A Protestant has no basis on which to condemn liturgical worship, especially given its place in the biblical tradition, which is why many Reformation traditions are still quite liturgical. (As it stands, New Testament scholars have identified various hymns and credal statements in Paul's writings believed to be widely used in apostolic Christian worship; the Synagogue relied upon a liturgical order; and of course, Paul encourages Christians to speak "psalms," that is, to rely upon the primary liturgical corpus of Old Testament worship [Eph 5:19]).
Of course, outside of scripture, the earliest Christian writings give us a generally liturgical portrait of Christian worship (e.g, the Didache, the Apostolic Tradition, etc.), reaching into the first century itself. The only exception to this rule is the charismatic spontaneity evident in the New Testament and in some early Christian sources (e.g., spontaneous anaphoras by "prophets" in the Didache.) Furthermore, modern liturgical scholarship has traced numerous elements of extant Christian liturgies to basic, even apostolic, core elements. It is safe to say, then, that extra-scriptural references, not unlike scriptural references, point Christianity in a liturgical direction. So far from being an undesirable form of Christian worship, then, liturgy is the natural, ancient, and universal reflex of Christian faith. But it is not its exclusive reflex. From youth services on guitars to grace at meals, family prayer circles, friends interceding for one another, prayers in distress, charismatic events, praise and worship adoration hours, and home worship groups (one of which I host), Catholics happily worship in spontaneous forms as well. In all these examples, I believe Catholicism strikes a quite appropriate, and biblical, balance--a balance also evident in mainline Protestantism.
However, one falls into an unbiblical extreme whenever one excludes liturgy as an authentic possibility for Christian worship, or worse, any vilifies the concept of liturgy altogether. Adventists are keen to encourage Christians to follow the example of Jesus, who worshipped in the Synagogue every Sabbath (Lk. 4:16). They would do well to remember themselves that at the Synagogue, Jesus was engaged in a manmade liturgy with prescribed elements and actions, out of which order all extant Christian liturgies have partly sprouted.