Via Motivated Grammar, I have found the very entertaining blog the ragbag, entertaining for its wide-ranging and whimsical discussions on literary topics, including etymology.
A recent post on words wholly related or words wholly unrelated states that host "bread consecrated in the Eucharist", host "army", and host "one who entertains guests" are wholly unrelated. This is true up to a point, but if you go further back it turns out that the second and third hosts are both derived from the Proto-Indo-European root *gʰos-ti- "stranger, guest, host". Host the army is from Latin hostis "enemy", from the "stranger" sense. Host as in someone who receives guests is from Old French hoste from Latin hospitem, hospes "host, guest, stranger, foreigner". Latin hospitem is from the form *gʰos-pot- (*poti- "master").
However, the ecclesiastical host is wholly unrelated.
guest is from the same root via Old Norse gestr. According to Watkins in the American Heritage Dictionary of Indo-European Roots, the PIE root meant "someone with whom one has reciprocal duties of hospitality", which explains how it could come to mean both "guest" and "host".