Example: "Their love was so strong that it was unspoken. Words were (replace) 'not necessary' for them to display their affection towards one another.
May be not the exact answer, however I think it could convey the message in the best way.
Words are unable to express their love.
They shared a tacit understanding. Words were gratuitous.
: expressed or understood without being directly stated
implied, implicit, unexpressed, unspoken, unvoiced, wordless
explicit, express, expressed, spoken, stated, voiced
: not necessary or appropriate
essential, indispensable, necessary, needed, needful, required
(Synonyms omitted -- none of the ones provided come close enough)
(From Merriam-Webster online)
Words were ________ (meaning not necessary). (idea: for them to display their affection towards one another)
Words were superfluous.
meaning "unnecessary or needless" -- Random House.
you may say that their love was indescribable or beyond description
- Indescribable means impossible to describe, because too intense, extreme, etc, for words.
you may also speak of an overwhelming love.
- Overwhelming means very great in number, effect, or force, i.e. extreme. it is used to describe something that is so confusing, difficult, etc., that you feel unable to deal with it.