In our 8 and 11 am services today we sang the hymn, "To Jordan's River Came Our Lord." The final stanza of the hymn struck me in a way it hadn't before:
Now rise, faint hearts, be resolute
This man is Christ, our substitute
He was baptized in Jordan's stream
Proclaimed Redeemer, Lord supreme.
A simple definition for substitute is: a person or thing that takes the place or function of another.
For being breakers of God's Law, we deserve God's punishment. Jesus, our substitute, takes our place and suffers our punishment. He suffers completely, becoming sin for us.
With our sin paid for, we are free to live as God's people. But it could never have happened with Christ, our substitute!