What does 'substitution' mean in regard to Christ's death and our death?
Substitution means Christ died in the sinner’s place to satisfy God’s law that the soul who sins must surely die. Yet believers still must experience that same death, not personally but in the person of Christ. God reckons them crucified with Him, so His death counts as theirs.
Les teaches that because the law says the soul that sinneth shall surely die, every human being as a son of Adam had to die. The loophole God provided was that Christ died in our place. But that does not remove the required death; believers still have to experience that same death in the person of Christ, and this is what is called substitution. It is one of the acts of God done on our behalf, and He alone accomplished it, reckoning believers as crucified with Christ.