When does one commit sin?

The principle of what differentiated temptation vs sin is laid down by Jesus himself in ,
Matt.5:27-28 “Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.” 
Here, “looks on a woman to lust after her“, is a deliberate and intentional mental action and not just an accidental momentary gaze on a temptuos woman. This expands sin of adultery to include mental imagination and concentration, not just physically acting it out. 
 
Moreover, James 1:14-15 explains the mechanism of sin crystal clear,
 “But every man is tempted, when he is drawn away of his own lust, and enticed. Then when lust hath conceived, it brings forth sin: and sin, when it is finished, brings forth death.”

Notice that sin starts with temptation which is defined as drawing away a person of his own lust and enticed. Lust, of and by itself, is not yet sin. Lust has to “conceive“, before one can be judged to have sinned. The etymology of “conceive” comes from the Greek word “syllambano” which means to “clasp, seize or capture as a prisoner, take hold“. Unless, the allure takes “hold“, it does not “bring forth sin …and death“. Lust has to “capture”, “seize” the person, i.e., he has to “dwell in it”, requires plan, imagine in his mind, strategize however long or short that plan may be, before it becomes sin “mentally“, or “actual physical act“.

Lust is temptation that we are drawn into and we must say No when we “think” about it or when it comes to our mind through our sensory system, and not “dwell” on it “mentally“nor “harbor” it, so it will not “conceive” or take “root“. Much less, take “physical action” on that temptation, as by so doing it is sin and results in death. Action, mentally and/or physically, to violate the God’s Law makes it sin. So, “flee” from it (1 Corinthians 6:18; 1 Corinthians 10:14; 1 Timothy 6:11; 2 Timothy 2:22).

Anyone who has the gift of the power tool of Holy Spirit to help us overcome evil, is given a “choice“, good or evil(Deut. 30:15-20). He is not forcing us (although it is a mandate)to choose between good and life vs. evil and eternal death. This “gift“, powerful Holy Spirit, is so awesome that Jesus has to be resurrected to receive it for us, cannot be forced on anyone to receive nor follow. 

Without Jesus being resurrected as promised, the Father could not fulfill another promise, which is to give him the gift of the Holy Spirit for us-ward. Walk in the Spirit and not towards the pull of the flesh. (Gal. 5:16-26) is an advice to all of us who are gifted with this power-tool. Hopefully, all with this gift will CHOOSE good instead of evil. Unfortunately and surprisingly, a few will choose wrongly. 
From Genesis, it has been a direction to us all, choose good instead of evil. But, even with choosing good, if this action is not out of following the guidance of the Holy Spirit, this good choice is still, as “filthy rags”. Our works have to emanate as “fruits of the Holy Spirit“, not from us devoid of the Holy Spirit’s guidance.Indeed, this is “hope-based” that everyone with this power tool will choose good over evil. Follow the direction of the Holy Spirit and do not grieve him (Eph. 4:30). Let us not constantly reject the guidance of the Holy Spirit, thus committing the “unpardonable sin”  (Matt. 12:31).

May God bless us all.