。
I think keeping promises is important.
Literal
Promise [obj-を] keep-[nominalizer-こと][topic-は] important thing [copula-だ][quotative-と] I [topic-は] am-thinking.
Stacks ことは… ことだ — two uses of こと, first as a nominalizer (keeping-promises) and then as a formal 'thing' (a fact/category). The double-こと gives a slightly bookish tone. Note the 私は being explicit and tucked near the end — Japanese often delays the overt 'I' for emphasis when contrast is implied, as if saying 'at least I (think so).'