By Calvinism, which has its roots in Augustinianism (which is rather different from the teaching of Blessed Augustine of Hippo), we mean the human tendency to despair. This is the tendency to see all as black, that salvation is impossible, whatever efforts we make – despite the Gospel saying that with God all things are possible – that depression is our life. It is the source of dour Scots, serious Swiss and earnest Dutch. This is the error that says that God has no mercy, only truth.
By Modernism, which has its roots in Pelagianism and Origenism, we mean the human tendency to self-exaltation. This is the tendency to see humanity as already saved, that no confession and repentance are necessary for forgiveness, that the effect of holy communion is magic, automatic, requiring no effort on our part. This is the source of modern humanism and secularism and, in the Church context, renovationism. This is the error that says that God has no truth, only mercy.
As ever, Orthodoxy, the faith in the God-man, transcendent and immanent, neither Monophysite or Nestorian, neither Origenist or Calvinist, has balance, truth and mercy, repentance and forgiveness, the Tradition of the Holy Spirit.