In 2013, Benedict became the first head of the Catholic Church to resign in seven centuries, amid speculation he was on the verge of a nervous breakdown, unable to cope at the top of an institution beset by scandals.
He said he no longer had the strength of mind or body to carry on, but his health appeared to improve after he stepped down and moved into a former convent in the grounds of the Vatican.
"In April, Benedict XVI will be 89 years old. He's like a candle which is slowly, serenely fading. He is serene, at peace with God, himself and the world," said Gaenswein.
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