I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose. Paul draws a sharp line here: either grace or law. Both together devalues the cross.
Clarity in thinking means: if you seek righteousness through your own performance, you indirectly say the cross was not enough. This is not theological hair-splitting, it is the core of the New Covenant.
Today: accepting grace means taking the cross seriously. Discarding grace means going back to the law.