Do I understand it correctly, the OP wants the phrase not to sound as an invocation or address? In that case using short form of adjective excludes the invocation/address meaning:
Бог великий! - Invocation/address "Great God!"
Бог велик! - Not an invocation, "God is great!"
The short form of the adjective as opposed to the full form has ...
он oбяжет сказать(/любить/прийти/etc)
This has meaning "he will make obliged somebody to do something" or
"he will order somebody to do something".
он долженствует сказать
Possible, but awkward. More often used to inanimate objects. Your desire is awkward, but it you really seriously want to use only verb for whatever reason, I suggest ...
Grammatically, this term is an adjective that has only short forms
Techincally, there is full form "должный" but it's not used like that.
I am not willing to use God's name with only an adjective that is not a proper finite present tense verb form, even if the adjective is a predicate adjective.
I admit that there could be a religious discussion ...