I have said that there are two opposite errors: one, the holding that salvation is not of God; the other, that it is not in ourselves. Now it is remarkable that the maintainers of both the one and the other error, whatever their differences in other respects, agree in this,—in depriving a Christian life of its mysteriousness. He who believes that he can please God of himself, or that obedience can be performed by his own powers, of course has nothing more of awe, reverence, and wonder in his personal religion, than when he moves his limbs and uses his reason, though he might well feel awe then also. And in like manner he also who considers that Christ's passion once undergone on the Cross absolutely secured his own personal salvation, may see mystery indeed in that Cross (as he ought), but he will see no mystery, and feel little solemnity, in prayer, in ordinances, or in his attempts at obedience. He will be free, familiar, and presuming, in God's presence. Neither will "work out their salvation with fear and trembling;" for neither will realize, though they use the words, that God is in them "to will and to do." Both the one and the other will be content with a low standard of duty: the one, because he does not believe that God requires much; the other, because he thinks that Christ in His own person has done all.
--Blessed John Henry Newman, “Righteousness not of us, but in us”, Parochial and Plain Sermons, Vol. 5