It should be explained that the difference between 'eved' (slave) and 'mesho'aved' (enslaved) is that an eved can possibly be in a situation where in practice he is a slave, but he is free in his thoughts and desires to be a free man. However, a mesho'aved is a slave in essence, who no longer wants to change his status. Egypt was a house of slaves, so whoever was a slave there was enslaved in thought and will as well. Therefore, when the Israelites left Egypt, they were redeemed in essence and in their very being, not just in action. This is also implied by the language of the verse (Isaiah 51:10): "Was it not you who dried up the sea, the waters of the great deep, who made a road in the depths of the sea for the redeemed to pass through," where 'redeemed' is a substantive noun, not just an adjective. This is the "new song that the redeemed sing," meaning redeemed in essence.