Jesus said some pretty harsh things during His earthly ministry, and, contrary to popular opinion, He didn’t always confine His harshness to the Pharisees. Consider this passage:
42 “Whoever causes one of these little ones who believe in me to sin, it would be better for him if a great millstone were hung around his neck and he were thrown into the sea. 43 And if your hand causes you to sin, cut it off. It is better for you to enter life crippled than with two hands to go to hell, to the unquenchable fire. 45 And if your foot causes you to sin, cut it off. It is better for you to enter life lame than with two feet to be thrown into hell. 47 And if your eye causes you to sin, tear it out. It is better for you to enter the kingdom of God with one eye than with two eyes to be thrown into hell, 48 ‘where their worm does not die and the fire is not quenched.’ ~~Mark 9:42-48 (ESV)
Jesus spoke these words to His disciples privately in one of their houses, just after warning John against pride and urging them to come to Him in childlike humility. He did not want them to cause others to sin, but He also wanted them to understand how seriously He would take their sin. And His words apply to His present-day disciples.
Of course Jesus didn’t mean Christians should literally dismember our own bodies. Earlier, He had told them that sin originates in the human heart (Mark 7:20-23), and I very much doubt that He wanted anyone to extract their own physical heart!
The clue comes from Colossians 3, where Paul spells out the implications of our new life in Christ:
5 Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry. 6 On account of these the wrath of God is coming. 7 In these you too once walked, when you were living in them. 8 But now you must put them all away: anger, wrath, malice, slander, and obscene talk from your mouth. 9 Do not lie to one another, seeing that you have put off the old self with its practices 10 and have put on the new self, which is being renewed in knowledge after the image of its creator. ~~Colossians 3:5-10 (ESV)
Rather than butchering ourselves, the Lord calls us to put our sin to death through the power of His Holy Spirit. This action can bring pain that is the emotional equivalent to amputating a limb or gouging out an eye. The Lord knew that dying to our sin nature would cause that sort of excruciating pain, yet He offered no mercy. Harsh? Absolutely! And He did not reserve that harshness for the Pharisees.