If we do not have a Savior who completed his work, then we do not have any hope for a complete salvation, for in that scenario, at least part of the finishing work would originate in us. But if we do not trust exclusively in the finished work of Christ, then we possess some measure of self-trust and that is shaky ground to say the least.
If you find yourself on shaky ground, the first category of self-examination is to look for any areas of self-trust. For example, if you trust that you are more righteous than other people; if you trust that you’re more righteous than you used to be; if you trust that you’ve memorized more bible verses than other people; if you trust that you attend a church service more than others and that you even are willing to listen to long, expository sermons. Can any of those things be perfect and completed in the sight of God? Of course not, but such is the case for Christ’s work on behalf of sinners. He is our only righteousness and salvation because those things were finished on that cross.