My understanding of the nature of the Holy Ghost - drawn from the Fourth Gospel primarily - is that he is the spirit of the ascended Jesus Christ.
The Holy Ghost can potentially be accessed by anybody, anywhere, at any time - and will provide us with the divine guidance and comfort of Jesus Christ... But
That is to say potentially.
In practice the Holy Ghost is only 'accessible' to Christians; because, to understand and respond appropriately to guidance and comfort, entails that we know the identity and motivations of the source of communication.
When a non-Christian accesses the Holy Ghost, seeks and receives a communication; then the guidance and comfort is Not acknowledged to be from our Saviour - but from some-one or some-thing else.
How a person does regard that source of communications would, presumably, vary. He might regard the communication as the voice of his conscience, memories of some earthly authority (parents, mentor, guru?), or coming from some other God or gods (like Socrates' daemon - who seems to have been an emanation of Apollo).
Yet, if we assume that the Holy Ghost really is from Jesus Christ; then to assume that HG guidance and comfort actually come from something else is a mistake.
Having mistaken the identity; we are then bound to misunderstand the communications: to misunderstand the authority and purpose of the communications.
For Christians; the guidance and comfort of Jesus Christ is 'designed' for our salvation; that is, so that we personally may follow Jesus to resurrected life in Heaven.
That purpose - and not something else - is what such guidance and comfort is for.
Because if we mistake the provenance of the guidance and comfort, then we will evaluate it wrongly.
For example, we might suppose it was designed to make us happier here and now, or be more successful in earthly life, or that it was supposed to gratify our desires and ambitions.
However, knowing that the Holy Ghost speaks for Jesus, we also know that the timescale of His communications stretches across eternity; and the source will therefore understand our specific present situation in this mortal earthly life in a Heavenly context - in terms of divine plans, meanings and purposes.