In order to have a sense of what is holy, one must have ideas and concepts related to holiness. In order for these ideas and concepts to be related to holiness, it must have roots in a religion, a theological framework, in which holiness as a concept is able to take root. If one’s religious framework has no Gods, there is nothing to consecrate. If there is no God or Goddess, no Holy Power to consecrate, then there is no holiness just as there is no profanity or things lacking in that consecration.
The very notion that an atheist can declare or recognize an image as holy is illogical on its face. An atheist framework is one in which there is no God or Goddess, and thus, no sacred. One may hold things reverently, that is, with deep respect, but without a religious framework that very concept that one may hold anything as holy has no basis. An atheist claiming to hold something as holy is a person claiming something to which one has no right by either religious framework or the result of one’s own philosophy.
If you eliminate the sacred from your worldview you cannot claim to hold onto something as sacred. Claiming symbols and symbol-sets as your own, when you have neither the investment in a religious framework, nor a religion itself that recognizes these things as holy, is appropriative from those whose identities revolve around the engagement with these symbols of Gods and Goddesses, and the Gods and Goddesses that are embodied by them. Using them in ritual, or for personal use, then, can be seen as a violation of communal bounds with those who worship these Gods and regard their symbols as sacred.
Rootedness in the Northern Tradition and Heathenry is what allows a person to understand Gods, the Runevaettir, the Ancestors, and vaettir of these religions. Rootedness in the Northern Tradition and Heathenry also allows for people, individually and communally, to understand what is held as sacred, holy, taboo, etc. There are plenty of worshipers of various religious backgrounds who come to worship Norse, Germanic, Scandinavian, etc. Gods without becoming part of Northern Tradition or Heathen religions. Yet, there is still rootedness in the Northern Tradition and Heathenry with many of these people. It is not any different than someone being on the outside of a religious community coming to a religious community seeking roots in their religion, practices, Gods, and spirits.
The idea of holiness and sacredness cannot exist without a religion and cosmology. One can study the myths as they like, even relate to the Gods, as characters, but holiness and sacredness and the treatment of the Gods, Ancestors, and vaettir in a respectful manner cannot be done in this fashion. Without this rootedness in the Northern Tradition and Heathenry, there is no context for the Gods, the Ancestors, and the vaettir as holy or sacred. Indeed, there is no context for Them at all. Sacredness, holiness, and similar terms, are the province of religions because religions are invested in the means by which holiness is understood, and the ways of determining what is holy, and held as sacred. Atheism cannot be invested in this understanding as it has no basis for holiness and the sacred, as atheism denies both on their face by its very outlook. Atheism denies that Gods exist, and in so doing, denies the cosmology They are rooted within. The notion of holiness within an atheist context, therefore, cannot exist.
Can people who are not rooted in the religious communities of Northern Tradition Paganism or Heathenry, then, be rooted in the Northern Tradition and/or Heathenry itself? Absolutely. There are plenty of folks in these religious communities without a community in their area to call home. Pagans tend to call these folks solitaries. Can people who are not rooted in the religious communities of Northern Tradition Pagans or Heathenry, then, be rooted in the Northern Tradition and/or Heathenry? Yes. They may be general polytheists without a community to call home or they may have a religious tradition to which they belong, but for one reason or another, are called to or brought into contact with our Gods, Ancestors, and/or vaettir. In either case, it is the acceptance of the Northern Tradition’s cosmology or one (or many) of the Heathen cosmologies, respect for the Gods, Ancestors, and vaettir, and understanding of holiness and the sacred that binds the Northern Tradition and Heathen paths together. For some Heathen paths, they will say that you cannot practice alone, and that you must have a community to call your own.
I do not take rootedness in the Northern Tradition and Heathen religions to mean that one cannot hold to relationships outside of these religions. It may mean that your rootedness in other traditions will need to be considered with or against the relationships you have in these Gods vs. Gods of another religion, and it may mean that you may need to consider the obligations you have to this communities vs. the other, but I do not think that Northern Tradition Pagans and Heathens are automatically precluded from relationships with Gods of other pantheons. What it does mean is that your considerations will need to be weighed, and priorities developed.