Hi,
my dad brought this beautiful calligraphy from Japan but forgot what it says / doesn't recognize it (speaks and reads a fair bit).
Grateful for your effort!
Looks like 一以貫之
Actually it is Chinese, a saying of Confucius, but also known in Japan. (Here is another example)
The meaning seems to be something along the lines of remaining consistent/faithful to your beliefs. (終始一貫信念)