at first - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Initially, at the start, as in At first the berries were green, but when they ripened they turned bright red. [Second half of 1500s]