to hold back from the harm, hardship, or action you have every power to carry out
This word always assumes power. Someone is in a position to strike, to destroy, to press an advantage, or to speak, and chooses to stop short. That is why it appears so often in the negative: the point of saying that God "did not spare" is that holding back was fully within reach.
The same restraint shows up in three settings across the New Testament:
- God withholding, or refusing to withhold, the judgment that a person or a whole world has coming
- a leader holding back a hard visit or a painful word to save someone the grief
- a speaker stopping himself from claiming more than he could rightly claim
It is worth keeping this apart from mercy (ἐλεέω), which names the help actively given to someone in trouble. Sparing is the quieter half of the same instinct: nothing is handed over, something is simply not done. A person can be spared and never know it.
When Paul writes that God "did not spare his own Son," he is reaching back to the story of Abraham and Isaac, where the Greek Old Testament uses this very verb for Abraham not withholding his son (חָשַׂךְ). The echo turns the sentence into an argument: the God who held nothing back at that cost will hardly hold back anything smaller.