The only limit to the realization of the future is our doubt about the present.
Literal
Future's realization-toward's only one limit [topic-は], present-toward's doubt [probably-だろう].
A quotation-style aphorism (paraphrasing Roosevelt). Two への ('toward X') noun modifiers set up the parallel structure: 実現への限界 ('limit toward realization') and 現在への疑い ('doubt toward the present'). だろう closes with a tentative assertive force — 'is, I suppose,' 'must surely be.' Not reaching for certainty, but for rhetorical thoughtfulness.