This discovery was to have a major effect on the treatment of heart disease.

We don't know whether the discovery really had the major effect on the treatmet or not, only with the sentence above?

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The discovery did indeed have a major effect as it was clearly stated in the statement.

But, I think you may be referring to the trustworthiness of the sentence. In that case, you are correct that we have no idea how true the sentence is.

The answer depends on what you are referring to. Please provide more information!