In his fascinating 1903 article "The Alleged Legalism in Paul's Doctrine of Justification," Geerhardus Vos explains the fundamental shift that took place at Paul's conversion/calling on the Damascus Road.
[I]n his conversion the pivot of his religious consciousness had been suddenly wrenched from [seeking of righteousness for man's sake rather than for God's] to the diametrically opposite position of a God-centered desire for righteousness. . . . In external appearance, indeed, the two were much alike, but, if ever, then here the external appearance was deceptive. What the two had in common was nothing more than their formal structure; in essence they lay as widely apart as the cult of self and true disinterested religion.
Vos is not contrasting those who forsake righteousness with those who seek it; he is contrasting two kinds of "seeking of righteousness," those who seek it for their own sake and those who seek it for God's sake. On the outside, they are difficult to differentiate. Internally, they are worlds apart. All will be made plain on the last Day.
--G. Vos, "The Alleged Legalism in Paul's Doctrine of Justification," in Redemptive History and Biblical Interpretation (ed. R. Gaffin; P&R, 1980), 396