Augustine of Hippo

He wrote with His finger upon the ground, as if He would indicate that people of the character of these men would be written on earth, and not in heaven, as He also admonished His disciples to rejoice that their names were written in heaven! Or, it may be that He meant to convey the idea that it was by humbling Himself (which He expressed by bending down His head) that He wrought signs upon the earth; or, that the time was now come when His law should be written, not, as formerly, on the sterile stone, but on a soil which would yield …

The Harmony of the Gospels, Book 4 ↗

Bede

His writing with His finger on the ground perhaps showed, that it was He who had written the law on stone.

Catena Aurea by Aquinas ↗

Thomas Aquinas

The Evangelist reveals the malicious intention behind those who were questioning Jesus when he says, They said this to test him so that they could accuse him. For they thought that Christ would say that she should be let go, so as not to be acting contrary to his gentle manner; and then they would accuse him of acting in violation of the Law: "Let us not test Christ as they did" as we read in 1 Corinthians (10:9). Then, Jesus checks his enemies by his wisdom. The Pharisees were testing him on two points: his justice and his mercy. …

Commentary on John ↗