For instance, if we share Christ to an unbeliever because we understand that we are accepted by God because of the finished work of Christ and that we have genuine concern and love for the unbeliever, then we are operating under grace. On the other hand, if we share Christ in order to gain acceptance from God, then we are walking under the law.
Similarly, two christians can go to church service on Sunday with two different motives - one christian loves to worship and exalt God because of his/her appreciation of what Jesus has done for him/her but the other goes because he/she has to go for fear that God will punish him/her or the cell leader or pastor will phone him/her up later in the week. The same goes for any other activity, like giving... , what are our motives? - only we can answer for ourselves.
Hence, we can see that both christians are involved in the same legitimate, very good christian activity, but with different motives. Legalism is insidious and can be detected only if we can discern the motives behind such activity.
When Jesus was on earth, He illustrated His higher motive and exposed the motives of the pharisees :
e.g.1 : Healing on Sabbath :
(Matthew 12:10-11, Mark 3:2) : Jesus asked - is it lawful to do good on the Sabbath? to save life or to kill?
e.g. 2 : the woes to the Pharisees
(Matthew 23:1-33) :Jesus exposed the Pharisees' hypocritic attitudes.
God's higher motive over the Pharisees have been to show his love, mercy, justice, mercy and faithfulness towards us. Love is the fulfillment of the law - this is exactly what Jesus demonstrated when He was on earth.