It is impossible for a priest to no longer “be” a priest. Ordination makes an indelible mark upon the soul of the man ordained that can never be erased. He is forever a priest.
He can leave active ministry. A priest can be released from clerical duties and obligations. However, this is separate from his promise of celibacy. Even once released from public ministry he is required to remain celibate unless separately released from that promise, which only the Pope can do (canon 291). This release from celibacy is not quick, easy, or guaranteed but it is possible.
