Truth, or eternal truth, encompasses everything that is true in the Universe, under God, etc.
When, however, one is asked, or forced, or proclaims, to speak the truth, it is always a portion of that big old truth; a segment pertaining to the matter at hand. It is the truth about something. About something. Some thing.
The truth about Bill Gates.
The truth about what really happened last night.
The truth about verismo opera.
The truth about yourself.
The truth about her (whoever she happens to be).
The (somewhat comical) line from John Keats' poem that goes "Beauty is truth, truth beauty" implies that all that is true in the Universe is beautiful, and vice versa: but even if it were, in fact, true, you would still have to be immortal, or nearly so, in your physical form, to speak all of it.
(Keats would have been correct, or nearly so, had he said "harmony" rather than "beauty," but that is besides the point).
The oath one takes in the courtroom in the United States and some other, less advanced, countries, "Do you solemnly (swear/affirm) that you will tell the truth, the whole truth, and nothing but the truth, (so help you God/under pains and penalties of perjury)?" is really not a request to speak ALL of it (which would take an eternity and then some to accomplish), but only the truth that is pertinent to the case.
Speaking truthfully, on the other hand, is less restrictive. Philosophically speaking.