While we are no longer under the law as far as practicing ordinances, tithing, and sacrifices, the moral law of God still abides. And the most important expression of the character of God in the law abides as well – self sacrifice.
Those dead in sin know the truth but they suppress it and continue in their sin.
The disciples thought the command to forgive was to difficult, so they asked for more faith. Jesus basically says you have the faith, use it. And in the end, just realize that you are only doing what you were told to do.
The world’s view of the love of Christ is based upon how His disciples treat and love one another.