Ejercicio verbos: pasado.Yesterday I went to Church.Ayer FUI a la Iglesia.Primera persona ¿Was,Went?
"I was at church" would use "estar", not "ser", so it would be something like "Estuve en la iglesia".
I remember thinking "whaaaaaaahhh?" when I found out that "fui" and "fue" mean both "was" and "went", but now that I (sort of) know when to use "ser", I think there's probably very few cases where it's ambiguous.
Here's an interesting short article about this: http://spanishlinguist.us/2013/03/spanish-was-went/