That does not decay; lasting, imperishable.(adjective)
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Use "undecaying" in a sentence
"From ancient Egypt to undecaying saints, we seem to have a fascination with the immaculate dead."
"Where the knife has touched them, they decay -- as that man's breast did -- but the rest of them remains undecaying year after year."
"Listen to me how Brahman, who is eternal and undecaying, and who is without beginning and without end, repeatedly creates and destroys all created objects."