。
I'm not saying I agree with you completely.
Literal
I [topic-は] you [with-と] completely same-opinion [copula-である] [quotative-という] [nominalizer-の] [copula-で] [topic-は] not.
~というのではない ('it's not that...') is a disclaimer structure — the speaker partially agrees but denies total agreement. 全く (まったく, 'completely') + 同意見 ('same opinion'). である is the formal copula. A nuanced, hedging statement.