“Free” does not mean there is not a cost to someone, somewhere. For example, if you get a free meal at a restaurant, then it may be free for you but it cost the owner of the restaurant money to pay for the food.
It is the same with the “free gift” of eternal life (Eph. 2:8). Eternal life is free to you and I who are Christians but it COST Jesus His life! In God’s justice, a perfect person had to sacrifice his life to atone for sins. The payment had to be made by a sinless person and only Jesus’ VOLUNTARY choice to give His life would suffice since all of us sin and fall short of the glory of God (Rom. 3:23).
Free to us does not mean “cheap” – there was a huge COST. Thankfully, God paid the debt for our sins with His own Son, Jesus.
-Mark (I owed a debt I could not pay, He paid a debt He did not owe to atone for all of my sins – and yours!)