* είσαι (eísai): This is the second-person singular form of "to be" (εἰμί, eimí). A Christian might use this if addressing one other person, e.g., "Are you feeling well?" would become "Είσαι καλά;" (Eísai kalá;). This is the most straightforward and common translation.
* είστε (eíste): This is the second-person plural form of "to be." Used when addressing more than one person, e.g., "Are you all ready?" would translate to "Είστε έτοιμοι;" (Eíste etóimoi;).
* είναι (eínai): This is the third-person singular form of "to be." A Christian might use this in a slightly more formal or distant way, or when referring to someone else, for example, commenting on a shared understanding: "Are they saved?" might become "Είναι σωσμένοι;" (Eínai sosménoi;). The plural would be "είναι" (eínai) followed by a plural noun.
* είμαστε (eímaste): This is the first-person plural form, meaning "we are." A Christian might use this inclusively, emphasizing shared faith, e.g., "Are we not brothers and sisters in Christ?"
The best translation depends entirely on the context of the sentence containing "are." Without knowing the sentence, είσαι (eísai) or είστε (eíste) are likely the most common and appropriate choices for a conversation between two or more Christians.