Q. Are Sinful Thoughts Equally Sinful with the Actual Deed?
I think the answer to your question comes from more than one text:
27 “You have heard that it was said, ‘YOU SHALL NOT COMMIT ADULTERY’; 28 but I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart” (Matthew 5:27-28, NAU).
13 Let no one say when he is tempted, “I am being tempted by God”; for God cannot be tempted by evil, and He Himself does not tempt anyone. 14 But each one is tempted when he is carried away and enticed by his own lust. 15 Then when lust has conceived, it gives birth to sin; and when sin is accomplished, it brings forth death (James 1:13-15).
The legalistic Jews of Jesus’ day interpreted the Old Testament Law in such a way as to find themselves guiltless – holy, if you would. Thus, you could look at a woman with lust, and not be guilty of adultery (when the law was viewed as they interpreted it). But Jesus took sin much deeper, so that the Old Testament commandments condemn all of us
19 Now we know that whatever the Law says, it speaks to those who are under the Law, so that every mouth may be closed and all the world may become accountable to God; 20 because by the works of the Law no flesh will be justified in His sight; for through the Law comes the knowledge of sin (Romans 3:19-20).
I think there is a difference between “committing a sin in our heart” (Matthew 5:28) and actually carrying out the sin by our actions. It seems to me that James recognizes this difference, so that he can speak of lust, but then go on to call it sin when that lust is “conceived” (James 1:15).
I think that James and Jesus agree on the fact that sin is the outcome of a sequence of events, not unlike the description of the foolish young man in Proverbs chapter 7. And while the legalistic Jews felt they were guiltless so long as they did not carry out the final act (adultery, murder) they were guilty. Jesus took them back to the roots of sin: lust (Matthew 5:27-30), and anger (Matthew 5:21-26).