If you are referring to the KJV of
Acts 14:15, the passage is not about the immutability of God.
14:15 "Friends, why are you doing this? We are mortals just like you, and we bring you good news, that you should turn from these worthless things to the living God, who made the heaven and the earth and the sea and all that is in them.
This was Paul's response to the crowd and leaders of Lystra who were proclaiming Paul and Barnabus as incarnations of Hermes and Zeus after a healing, and wanted to offer sacrifices to them.
14:11 When the crowds saw what Paul had done, they shouted in the Lycaonian language, "The gods have come down to us in human form!"
Paul's response was a strong denial that they were gods but that they were there to bear witness to the creator God. In the Greek of that verse there is no word "passions." The Greek phrase is "we are men like you." In context, that is in contrast to being Hermes and Zeus.
As far as God being immutable and impassible, those are both defined
a priori by the categories of Greek philosophy in which they were originally cast. They are not ideas presented by Scripture, since it repeatedly describes God as both changing and feeling. While some would argue that those characteristics are only anthropomorphisms (attributing to God human characteristics), they are no more so than trying to define God in terms of logically derived Greek philosophical categories.
The reality is that we have no way to know what God is not (not changeable, not passionate), yet we do have biblical witness to him interacting with humanity both in terms of change and in terms of care and love for humanity. The simple idea of relationship suggests both interaction and the possibility of both pain (rejection) and pleasure (love). While those cannot be defined in terms of human physical or physiological emotion, neither can they be ruled out by logically defining God in such a way that excludes them.
I'd go with the biblical witness to God, and leave some ambiguity, rather than accepting tightly defined logically constructed categories developed from the assumptions of Neoplatonisn through Augustine. I would suggest that the latter is an example of defining God in terms of what we think God ought to be to be God. That ends up with a much smaller God than we realize since it fits within our human understanding.
Grace and Peace,
Dennis B.