Remission is not the same as forgiveness. Repentence brings forgiveness, Baptism brings cleansing, Spiritual birth brings power to remain clean.
To use the same analogy Christ used in
Matthew 12:43-45 and
Luke 11:24-26, an evil spirit was cast out and after a time returned to find the "house" swept (repented), and garnished (baptized) but empty (not filled with the Holy Ghost). That spirit went and invited 7 more spirits even more evil than himself and started a commune.
Sweeping is good to keep the "cobwebs of sin" down, but baptism washes out the spider and the Holy Ghost keeps him from returning. Therefore, all three steps are crucial to becoming and remaining saved.
Another, more direct analogy would be this: Repentance stops <insert sin of your choice>, but could leave the <vehicle of aforementioned sin> in one's house. Baptism throws the <vehicle for sin> away. The Holy Ghost gives the power to resist accessing another <vehicle>.