The alternative is just as much of a 'just because'
David Bentley Hart's The Experience of God does a pretty good job of laying this out from a religious perspective: https://www.amazon.com/Experience-God-Being-Consciousness-Bl...
Unfortunately, the book is in some ways kind of a lengthy introduction to what it actually purports to be, but it does a needed job nevertheless.
Anyways, interesting about this Flew guy. Few people change their mind about anything--let alone core beliefs like this.
And I think where he actually landed also offers a pretty good example of what should be obvious: that religion as we know it should not be equated with belief in God, and that one can easily believe in some form of "God" without accepting the postulates of various religions.
https://www.amazon.com/Experience-God-Being-Consciousness-Bl...
I haven't read this one, but seems on point: https://www.amazon.com/Atheist-Delusions-Christian-Revolutio...
To be clear, Hart's own religious views seem pretty eclectic, and he is much less dogmatically Christian than blurbs and marketing of his books suggest.