Donald Fairbairn ( Life in the Trinity: An Introduction to Theology With the Help of the Church Fathers ) writes, “In the mind of the early church, impassibility implied that God could not be adversely affected or damaged by anything we might do.  We cannot ruin the fellowship within the Trinity or disrupt the purposes of God or cause his will to fail.”

This is why the church fathers think of impassibility in connection with timelessness.  Timelessness does not mean that God is incapable of “entering” time and interacting in time.  He obviously did interact in time; that’s what incarnation is about: God the Word living a human life, which means a temporal life.

It means, on the one hand, that He is not damaged or derailed by the ravages of time (as we are), and that, since He always already possessed all fullness, He cannot become more than He was through time.