Does God Change His Mind? Yes and No, Says Tertullian
The nice thing, though, about obvious problems in Scripture is that other people have seen them too. Here’s Tertullian (Contra Marcion, 2:24), writing around AD 208:
Samuel says to Saul, “The Lord has rent the kingdom of Israel from you this day, and has given it to a neighbour of yours that is better than you, and into two parts shall Israel be divided: for He will not turn Himself, nor repent; for He does not repent as a man does” (1 Samuel 15:28). According, therefore, to this definition, the divine repentance takes in all cases a different form from that of man, in that it is never regarded as the result of improvidence or of fickleness, or of any condemnation of a good or an evil work.
What, then, will be the mode of God’s repentance? It is already quite clear, if you avoid referring it to human conditions. For it will have no other meaning than a simple change of a prior purpose; and this is admissible without any blame even in a man, much more in God, whose every purpose is faultless. Now in Greek the word for repentance (μετάνοια) is formed, not from the confession of a sin, but from a change of mind, which in God we have shown to be regulated by the occurrence of varying circumstances.
In other words: God doesn’t change his mind like we do (which is the point of v28), but he does in the sense that he responds differently to different circumstances (v35). Simple as that.