I always say at the onset to those who make the incorrect uninformed assertion regarding the triquetra symbol being derived from Wicca, paganism, etc. that that calendar in your office and/or house and/or in the rooms of your children, — are filled with symbols of pagan gods (all days and names of months were named after pagan gods). Thus, any objection to the triquetra as use by Christians would be inconsistent and historically ignorant lacking any meaningful basic research on the triquetra and its origins in religious and non-religious usage.
In terms of the triquetra (Trinitarian symbol), you should not base an argument on ignorance, and unaccredited internet articles. Primarily, KJV Onlyists and anti-Trinitarian groups (esp. JWs, and unstudied Oneness advocates) chiefly utilize the pagan-triquetra arguments against it. So Christians should strive to do the objective research, in order that they not provide bad untruthful arguments and appear unread. In point of fact, The triquetra is a very old symbol and dates back perhaps to around 500 BC. But its actual origins are unknown. Some scholars believe it to be Celtic in origin, and it is sometimes called the Irish Trinity Knot.
The triquetra symbol is also found in Norse Viking artifacts such as combs and saddles; found on a Norwegian coin from around the 11th cent.; and there is a Japanese form, again with no religious significance. Further, the triquetra has been found on Indian heritage sites that are over 5,000 years old; found on carved stones in Northern Europe dating from A.D. 8th cent. as well as found on early Germanic coins-with no religious significance at all. It is certainly possible that various cultures developed the basic design arrangement independently. But in spite of where or when it first appeared, it has been associated to a vast number of meanings through time.
However, to early Christians (and many today), the triquetra symbolized the Trinity (one God, three persons). For example in the late 8th cent. Book of Kells was an exemplified manuscript book in Latin containing all four Gospels together with various prefatory texts contained also figures of triquetras. The triquetra symbol has been found in Norwegian churches dating to the 11th century.
In conclusion, the triquetra has been used historically by all kinds of groups to mean different things. As with other Christian symbols and Christian holidays (e.g., Christmas, Easter, cross, etc.), we embrace the Christian significance—not its origin. In spite of the (unclear) origins, the triquetra has a rich meaning that has been used by the early church to signify the Trinity. No Christian used it as a pagan symbol, in the same way no Christian uses a calendar today on their wall to exalt the pagan gods of the days and months it represents—thus, calendars were factually derived from pagan in origins.
Historically, for Christians, the Triquetra represents the Trinity, not its supposedly pagan origins. And those who object (due to a mass of misinformation) to this Trinitarian symbol, since they do not have a problem with pagan-origins calendars in their homes, do they have a problem with the Apostle Paul’s quotations of pagans writers to make a biblical point, viz., Epimenides of Crete in Titus 1:12 and Acts 17:28 (referring to Zeus); Aratus of Cilicia in Acts 17:28 (also referring to Zeus); and Menander in 1 Cor. 15:33?
In point of fact, for hundreds of years Christians have been using the triquetra a symbol that proclaims the doctrine of the Trinity.