When Jesus saw that the equally guilty man was not present, He knew their motive. Further, He knew the man must be of some importance, influential in the community or else the man would also now be before Him. Moreover it is quite possible that the man was himself one of the leaders - having deliberately seduced the woman thereby "sacrificing himself" to commit the act as part of a conspiracy for the very purpose of entrapping Jesus. "But what sayest thou?" that they might have something with which to accuse Him, they inquired. Thus, the real issue before us is actually that of "authority" (cp. verse 36!).
It is most important that the reader realize that Jesus did not set aside the Laws of God or make an exception with this woman as though God had changed His mind or had "softened" from the Old Testament to the New Testament - that God was a God of wrath in the Old but had somehow "evolved" into a God of love, grace, and compassion in the New. God loved and had compassion on the exposed adulterers all throughout the Old Testament. He certainly did not love or feel more compassion for her than any before her. It was always the sin itself that He hated, but His holy nature and justice then as now, called for righteous judgment and punishment. God never changes (Mal.3:6).
First, this was still the time of the Old Covenant. The New Covenant could not come into effect until the required blood of the Covenant was shed. But the reader must come to see that Jesus perfectly upheld the demand of the Law - Jesus actually told these religious unbelievers to stone her (verse 7)! He told them to obey the Law - but dealt with their consciences, bathed in murder as they were, by the prefacing remark "He that is without sin among you" let him cast the first stone. The idea behind this stipulation was twofold. First, Jesus caught them unawares in that rather than having the "Bible study group" carry out the stoning, Jesus called on the unregenerate scribes and Pharisees to perform the deed. Thus if they so did, it would be they whom the Roman authorities would come against and not Jesus. They would have fallen into the pit that they themselves had dug (Pro.26:27). The Romans had taken the power of life and death away from the conquered Jews (Joh.18:31), and Roman law did not condemn an adulteress to be put to death.
In the second place, Jesus is challenging them to merely obey the law to which they so devotedly cleave. Jesus is calling on the required two or three eye witnesses (Deu.17:6-7) to now step forward. If they are credible witnesses, they must now identify themselves and also make known the identity of the man. If they will not identify the man they will be disobeying the law and thus will incur guilt. The man having been summoned, the stoning could continue but the first stones must be cast by these same men.
The qualifying "without sin" in Scriptural context with regard to witnesses, does not mean "moral perfection" as many suppose, thereby creating a problem here that does not exist. The context refers to the witnesses not being guilty of sin with respect to their being false or unrighteous witnesses in the matter at hand (cp. Lev.20:10; Deu.17:6-7; Exo.23:1-2 & 7; Deu.19:15-19 and Pro.6:16-19). This is especially made clear in Exodus 23:1-2, 7. The Deuteronomy 19 passages continue the theme of dealing with false witnesses by God's charging the judges with the responsibility of having the sentence that would have been applied to the accused meted out to the false witness. The implication from Jesus' stipulation is that if they obey God, being innocent and without sin regarding this matter, God would doubtless protect them from the Roman authorities. If, however, they are not - well then, they could not expect to be so delivered could they? They would thus incur the same penalty.