Answer: a combination of words that may almost be dangerous, and thus quite tempting for many. The problem arises from the fact that even the eternal is time, in this that it pretends to everlasting presence. Truth, if well defined, refers itself to a kind of reality that should situate itself outside of time, consequently it is without time referential, and the combination of both words could be considered ‘absurd’ (contradiction in essence of character), if you keep yourself very strictly inside of definition.
The only valid combination of such obtuse concepts may be ‘God is eternal’, if and only, you consider the immanence of God inside of nature, and if you accept that nature or creation is everlasting (Psalm: « his creation was made to last »). But truth is not an active, immanent principle, as it is in fact, from a subjective point of view: the adequate expression of what is at its very moment (Plato), resulting of the relationship in probable synthesis of a human mind with what can be called ‘spirit’, this without, there is truth none. Which is to say that the concept of ‘truth’ is born from the aceptation of this relationship, more or less well expressed on the other hand, and disappears as concept if the human does not accept the concept of spirit as such.
To give a clear example: You say, for example ‘God is’ is an eternal truth. How though? Even if the fact were, the truth does only appear if it switches some little light in a human mind. If you consider the possibility that no human at a certain moment may arrive to such conclusion, the eternity of such a ‘truth’ is shortcut by human blindness. And, if God were to maintain such an evidence, you can be sure he speaks a different language which makes his own belief in himself be of exception.