2:7a No, we speak of the mysterious and hidden wisdom of God,

No, the wise message we (excl.) tell is God’s. It concerns his secret plan which he did not previously reveal to mankind/anyone.
-or-
Instead, we (excl.) talk about the wise plan that God formed/made. Until now no one knew about it because God did not make it known/reveal it to anyone.

2:7b which He destined for our glory before time began.

We (excl.) tell about God’s plan to give us (incl.) a glorious/wonderful life. Before God made the world he decided to do this for us.
-or-
It is what God long ago decided to do even before he started to make anything. And it includes all the surpassingly good things he intends to do for us (incl.).

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