We needed forgiveness because we were sinners and our sin was so great that we would have been eternally cut off from Father God were it not for Jesus dying on the cross for the forgiveness of our sins.
Having said that, though we have been forgiven once and for all,our sinful nature makes us prone to sin repeatedly. This is where obedience comes in. When we obey and work at obeying we minimise our falling into sin. Obedience helps us to walk in the leading of the Lord, and it is obedience which keeps us in right relationship with God.
With only forgiveness we may be restored in our relationship wtih God but we are still stumbling along blindly in our daily living. With only obedience it is still possible for us to fall into sin because of our inherent imperfections, requiring forgiveness. Hence we obedience and forgiveness are a package that we need.
We are saved by grace rather than works because God's standards are very high and all our works amount to nothing.(our righteousness is like filthy rags)We can never attain His standards no matter how hard we try and God knows this, yet in His infinite love for us He extends to us grace far beyond what we deserve so that we may know beyond a shadow of doubt that we are saved by HIS grace and not our good works. This is also to keep us humble, because if we were saved because of our works it wouldn't take us very long to get puffed up in pride. (God is so good!! He knows our propensity to pride,and therefore He will not give us reason to be proud)