WHY TO WE TOUCH LEFT THEN RIGHT WHEN WE MAKE THE SIGN OF THE CROSS?

QUESTION? / COMMENT

Some years ago while working as an AmeriCorps VISTA, a non-Catholic colleague asked me something that I didn't have an answer to. It was near the end of our year of service, so I never got back to the colleague before we parted ways. But the question has always been in the back of my mind since then. I've researched online as best I could, but all I can find are articles/videos on how to do the sign of the cross, never an explanation of why. I emailed Relevant Radio some weeks ago but have yet to hear a response. Or if someone did respond, I wasn't listening at the time. Hence why I'm emailing y'all today.  :) 

When we do the sign of the cross, it's...

           In the name of the Father (touch forehead) and of the Son (touch the chest) and of the Holy Spirit (touch the left shoulder, then the right shoulder) Amen.

The question that my non-Catholic colleague posed is why do we touch left then right? My follow up questions are: is there an historical significance to touching left then right? Is it considered sacrilegious to touch the right then left shoulder? Do you have any links online or books to further explain this? Bottom line: is there a sin in not going left to right? 

ANSWER! / COMMENT?

In my book "Lord prepare my hands for Battle" https://jesseromero.com/store/lord-prepare-myhands-battle-book I cover that question extensively. Its on p.21-25.