Theses words were a tall order for the Israelites during His earthly ministry.
Let us never forget, however, that Jesus as the prophet greater than Noah, Moses, and Samuel was restoring the law to a pristine, impossible wholeness.
Then He drops this command: "Take no thought for your life."
Now, why can we relax and no longer worry about our lives?
Well, let's establish this truth up front: man is born dead in his trespasses.
Adam and Eve died the moment they ate from the tree forbidden to them.
"But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die." (Genesis 2: 17)
Paul then explained what happened to all mankind through Adam:
"Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned: 13(For until the law sin was in the world: but sin is not imputed when there is no law. 14Nevertheless death reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression, who is the figure of him that was to come." (Romans 5: 12-14)
So, man is not just in sin - he's dead!
Why would dead people take thought for their lives if they have no life to begin with?!
Jesus declared the greatest hope that He gives to us:
"The thief cometh not, but for to steal, and to kill, and to destroy: I am come that they might have life, and that they might have it more abundantly." (John 10: 10)
His life is not bad, but good!
"For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ." (Romans 5: 17)
Jesus does not just give us life, but we reign in life - as kings and priests! (1 Peter 2: 9) in Christ Jesus!
He gives us all things richly, too (Romans 8: 31-32)
Let us not be surprised today by the commandment which Jesus gave to the Israelites during His earthly ministry. We had no life then, but today He is our life:
"If ye then be risen with Christ, seek those things which are above, where Christ sitteth on the right hand of God. 2Set your affection on things above, not on things on the earth. 3For ye are dead, and your life is hid with Christ in God. 4When Christ,who is our life, shall appear, then shall ye also appear with him in glory." (Colossians 3: 1-4)