If it is the case that.../ If it is true that...
Used to express that if a certain condition is met, then something follows; often used in formal settings.
Examples
彼がそう言うとあれば、私も信じるしかない。
If he says so, then I have no choice but to believe him.
成功するとあれば、一生懸命に取り組むつもりだ。
If success is guaranteed, then I intend to work hard for it.
彼がその条件を受け入れるとあれば、話し合いの余地がある。
If he accepts those conditions, then there is room for negotiation.
Quick check
彼がそう言う___、私も信じるしかない。