It is the Language of Scripture, that Christ in us is our Hope of Glory; that Christ formed in us, living, growing, and raising his own Life and Spirit in us, is our only Salvation.  And indeed all this is plain from the nature of the Thing; for since the Serpent, Sin, Death, and Hell, are all essentially within us, the very Growth of our Nature, must not our Redemption be equally inward, and inward essential Death to this State of our Souls, and an inward Growth of a contrary Life within us?  (William Law; emphasis in the original)

But those things which proceed out of the mouth come from the heart, and they defile a man.  For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies.  (Matthew 15:18-19)

William Law is pointing out that the sinner was on the road to receiving his wages of death and bore in his own nature sin, death, and hell.  The inner life of the sinner does not bear the image of God and is not able to come into a relationship with the Lord.  This person needs an inward work for an inward problem.

For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.  (Romans 6:23)

Before Christ can begin His inward work the sinner becomes a saint by believing and trusting in the death, burial, and resurrection of Jesus Christ as redeeming him with the payment of Christ’s shed blood upon Calvary.  Then the Holy Spirit changes us from the inside out.  Christ is then in us.  The work has begun; our salvation is secure.  The process of sanctification is then begun with Christ being formed in us until the outside displays what has been accomplished on the inside.  The contrary life that Law speaks of is the life we are given in Christ at the moment of our conversion: we are salt and life in a world of flavorless darkness.  The world can see me only a little at first, but as Christ is formed in me the world sees more and more the light within me, the life of Christ.

You are the salt of the earth; but if the salt loses its flavor, how shall it be seasoned? It is then good for nothing but to be thrown out and trampled underfoot by men.  You are the light of the world.  A city that is set on a hill cannot be hidden.  Nor do they light a lamp and put it under a basket, but on a lamp stand, and it gives light to all who are in the house.  Let your light so shine before men, that they may see your good works and glorify your Father in heaven.  (Matthew 5:13-16)

Then Jesus spoke to them again, saying, “I am the light of the world. He who follows Me shall not walk in darkness, but have the light of life.”  (John 8:12)

By this we know that we abide in Him, and He in us, because He has given us of His Spirit. (1 John 4:13)