The great theologian B. B. Warfield on why the gospel is needed for believers:
There is nothing in us or done by us, at any stage of our earthly development, because of which we are acceptable to God. We must always be accepted for Christ's sake, or we cannot ever be accepted at all. This is not true of us only when we believe. It is just as true after we have believed. It will continue to be trust as long as we live. Our need of Christ does not cease with our believing; nor does the nature of our relation to Him or to God through Him ever alter, no matter what our attainments in Christian graces or our achievements in behavior may be. It is always on His "blood and righteousness" alone that we can rest.