The Sinless Savior Became Sin.
For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him. 2 Corinthians 5:21 NKJV
The purity of the Mediator: He knew no sin. The sacrifice he offered: He was made sin; not a sinner, but sin, that is, a sin-offering, a sacrifice for sin.
The end and design of all this: that we might be made the righteousness of God in him, might be justified freely by the grace of God through the redemption which is in Christ Jesus. Note, As Christ, who knew no sin of his own, was made sin for us, so we, who have no righteousness of our own, are made the righteousness of God in him.
Our reconciliation to God is only through Jesus Christ, and for the sake of his merit: on him therefore we must rely, and make mention of his righteousness and his only. - Matthew Henry