For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life. For God did not send His Son into the world to condemn the world, but that the world through Him might be saved.
He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God.
John 3:16-18 NKJV
Within this promise is a condition that must be met to receive it. Believing. The offer is given to all of us and by grace we are given the opportunity. But in order to avoid condemnation and receive eternal life, we have to believe in our Savior. Much like one who is being saved from drowning, we thrash and flail making our rescue more difficult for the one who has jumped in to help us. But at some point, we have to trust and believe we are receiving help. If we don't, we are already condemned to drown. The first step is accepting that God sent us a Savior because He already knew we couldn't save ourselves. The second step is believing in who He is.