Translation:We used to be religious, but now we are secular.
James the confusion is the double yud. But here it isn't a dual plural. Its a plural adjective. דתי gets ים. And then it looks like it could be dual but it isnt. There are a fairly limited number of dual forms left in Hebrew most of which are naturally occuring pairs such as body parts, glasses, pants, scissors, bikes, etc. And certain time words like מחרתיים, the day after tomorrow. Or שנתיים two years. Or פעמיים two times/twice.
In Hebrew, generally the past tense verb does not require the pronoun, but the present tense does. Why? Because the requirement depends on whether each verb conjugation has its own pronoun or not. The present tense plural verb in our sentence isn't there at all (in English we would say it is understood; Modern Hebrew does not use the present tense to be forms), but the adjective it refers to here, חילונים, can refer to we, the plural you, or they and therefore the pronoun must be included for clarity. The past tense היינו, which only refers to we, need not include the pronoun אנחנו.
I hope this helps!