Initially, at the start, as in At first the berries were green, but when they ripened they turned bright red. [Second half of 1500s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
Examples from the Web for at first
Even Mr. Seward, cautious and conservative diplomat as he was, at-first "opposed any concession or surrender of the prisoners."The Every-day Life of Abraham Lincoln
Francis Fisher Browne