Strong's 3801 (ὁ ὢν καί ὁ ἦν καί ὁ ἐρχόμενος)

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which art (is, was), and (which) wast (is, was), and art (is) to come (shalt be)

the one being and the one that was and the one coming, i.e. the Eternal, as a divine epithet of Christ

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a phrase combining G3588 (ὁ) with the present participle and imperfect of G1510 (εἰμί) and the present participle of G2064 (ἔρχομαι) by means of G2532 (καί);