Solo fidelis (through faith alone) is a heresy. Mortal sin kills the soul and makes one deserving of damnation. Only through the grace of The Lord and through the forgiveness of The Lord Christ can we be restored in a state of grace and be accepted into His Kingdom when He decides our time comes.
Part of God's image that we reflect is His free will. We can and do choose whether to accept His will, or deny Him. This is one of His greatest gifts, but since we are only the image, and imperfect at that, we can and often do choose to deny Him. Only through His Grace are we gifted the strength to resist sin and align our will with His.