Do you think there is ever a time when the spoken word of God could be of less effect than no word at all?
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I do not think the least hint in that direction.
The Scripture is clear about itself on that score.
God's Word is powerful--He spoke all that is into existence.
Methinks the title of the fine book . . .
YOUR GOD IS TOO SMALL
would fit a lot of the perceptions and constructions on reality which we read from the naysayers.
Amen! And that is no coincidence. God gave men the ability to speak in order to preach His glory and praise His name.
And it's through ears given by God that men know the truth of the Gospel. As you referred to Isaiah...
"So shall my word be that goeth forth out of my mouth: it shall not return unto me void, but it shall accomplish that which I please, and it shall prosper in the thing whereto I sent it." -- Isaiah 55:11