fervently, zealously, devoutly, ardently
Она и́стово молилась.
She prayed fervently.
Он истово трудился на этой работе
He worked with great dedication at this job.
Она истово молилась перед иконой каждый вечер
Every evening, she prayed fervently before the icon.
Старик истово верил в свою правоту
The old man fervently believed that he was right.
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