I think, therefore...

Descartes thought he had it all figured out.
"I think, therefore, I am."
If he can think, he deduced, he must exist, and thereby he established an intimate connection between thinking and existence, a correct one as far as it goes.

However, Descartes didn't deduce the depth of the connection between thought and physical reality.  When one considers God, rather than man, thought and reality become essentially the same thing.

And God said, Let there be light.--Genesis 1:3
And God said, Let there be an expanse between the waters.--Genesis 1:6
And God said, Let the land produce...--Genesis  1:11and so on.

Remember, our Creator doesn't have a mouth.  When God said, He thought.  His saying is done as effortlessly as thinking.  He thought the world into existence--the whole heaven, the whole earth.
There is no separation or hesitation between what God thinks and what happens.
There wasn't then; there isn't now.

 Now, with that in mind, consider this:
He chose us in Him before the creation of the world to be holy and blameless in His sight. In love He predestined us to be adopted as His sons through Jesus Christ...--Ephesians 1:4-5

Here as in the very act of creation, no pause exists between God's thought or will and its accomplishment or reality.  If He chooses, it happens at the same moment that the choice is made.

It is different for us, and a good thing, too.
Imagine if everything we thought actually happened--Yikes.
No, to make stuff happen we, as humans, have to DO something.

God has already both purposed and accomplished salvation.  We, however, have to act.  We not only have to know we can be saved, we have to turn the thought into deed:
If you confess with your mouth "Jesus is Lord" and believe in your heart God raised Him from the dead, you will be saved--Romans 10:9
Everyone who calls on the name of the Lord will be saved.--Romans 10:13

Descartes got it partly right.  He existed, as do we, not because he thinks, but because God thought.
We know salvation for the same reason.
But our humanity does not wrap around that, so we join with God's will the only way we can--by both belief and the action of consent.
We say, "Yes, Lord."