Circumcision of male children was done by Abraham and his descendants in response to God's covenant (Genesis 17:1-14).  It marked each generation as those who were under God's authority and bound to obey His laws.  Circumcision was also a life-long sign that they were set apart, cut out from the rest of humanity to serve the Lord, only to marry within His family and to raise children who would continue in the faith.  However, although the physical surgery left a permanent mark on the body, it had no effect on changing the heart (Jeremiah 4:4).  
 
But faith in Jesus Christ is very different.  When we repent of our sins and accept His rule in our lives He does not just give us a set of moral rules, but He performs surgery on our hearts (the place of our deep desires, decisions and will) (Ezekiel 36:26).  We cannot get rid of our sinful nature by ourselves; we need Christ to mark out all that has to go and give us a desire to be cut free from it.  We cannot make ourselves right; we need Him to take away all that spoils and replace it with all that shines.

Baptism is a sign that we accept His right to do this surgery on our hearts, because we have put our faith in Him and we belong to Him.  In the waters of baptism, we identify with Jesus Christ in His death, burial and resurrection (Romans 6:4).  We count our old life as dead and buried as we go under the water; as we come out of the water we accept His promise of being a new creation with a new life (2 Corinthians 5:17).  Baptism is a physical sign of what God will do if we desire Him to do it (Philippians 2:13).  The willingness to be baptised is the willingness for God to circumcise our hearts.   
 
Quite rightly, the Bible encourages us to cooperate with what Christ is doing in our hearts.  But however much we discipline ourselves (and we need to), the transformation of our lives is not achieved through our own initiative or personal effort: it is the work of Christ.  Without Him we can do nothing of eternal value (John 15:5).  But as we allow His authority to operate in our lives, we will see wonderful changes which we could never have achieved by ourselves.  It is those transformations of life and character which are the living testimony to the power and grace of Christ (2 Corinthians 3:2-3).  Every piece of evidence that Christ is really at work changing lives, supports the proclamation of the gospel - giving others a desire to know how to put their faith in Jesus Christ.  So let Him do His work in you today.