There are no grounds, as some argue, to think repentance doesn’t sit at the front of the Pauline argument. Schnabel convincingly argues

• There is no basis for the argument that Paul avoids the terms μετάνοια /
  μετανοεῖν because of antipathy to the term devalued by the penitential practices
  in contemporary Judaism, as Johannes Behm alleges, or merely because
  for Paul “μετάνοια is comprised in πίστις, the central concept in his doctrine of
  salvation,” or because this term “did not stress sufficiently God’s action in
  salvation.” The relative scarcity of the terms μετάνοια / μετανοεῖν in Paul’s
  letters should not be accorded too much weight. Inferences from word statistics
  are, at least in this case, not helpful. Paul does use the terms μετάνοια /
  μετανοεῖν. He also uses other terms and phrases in order to express the need
  to, and the reality of, changing mind and heart, outlook and behavior. We have
  seen that Paul knows the Jewish doctrine of repentance, that his missionary
  preaching calls for repentance, that his theological discourse presupposes
  repentance, that his rhetorical discourse in his letters includes the discourse of
  repentance, and that his ethical discourse entails exhortations to repentance.
  Christians are people who have repented of their sins and who have committed
  themselves to living for God, and they are people who continue to repent of
  sins they find themselves doing, continuously committing themselves to doing
  the will of God... [There is a] continued necessity of exchanging
  secular standards for the standards of the will of God, changing the patterns
  of perception concerning what is acceptable behavior, regretting false
  thinking and wrong behavior, returning to faith in God and in Jesus as the basis
  for proper Christian behavior, all the while relying on the power of God and his
  Spirit who continues the transformation of the justified sinner (Schnabel, Novum Testamentum 57 (2015) 159-186)

It’s just common sense. Christ on the cross disarmed the power of Satan leading people to God, but this is of no effect if people don’t have a change of heart. Adherents to the penal substitution interpretation of the cross are reduced to the absurdity that the cross accomplished everything but accomplished nothing, since Satan and sin still reign until Jesus returns. All of this remains opaque until we contrast penal substitution with the moral influence interpretation of the cross, which I do here:

A Critique of the Penal Substitution Interpretation of the Cross of Christ

Jesus Mythicism: Moral Influence vs. Vicarious Atonement—and Other Problems