Bonar makes a point early on in the book that is critical regarding salvation by faith. It's been repeated again and again in various ways, but his statement still rings forth with a brilliant tone:
With a weak faith and a fearful heart many a sinner stands before the altar. It is not the strength of his faith but the perfection of the sacrifice that saves; and no feebleness of fiath, no dimness of eye, no trembling of hand, can change the efficacy of our burnt offering. The vigor of our faith can add nothing to it, nor can the poverty of it take anything from it. Faith, in all its degrees, still reads the description, "The blood of Jesus Christ his Son cleanses us from all sin." If at times the eye is so dim that it cannot read these words, through blinding tears or bewildering mist, faith rests itself on the certain knowledge of the fact that the inscription is still there, or at least that the blood itself (of which these words remind us) remains in all its power and suitableness upon the altar, unchanged and uneffaced. God says that the believing man is justified: Who are we, then, that we should say "We believe, but we do not know whether we are justified?" What God has joined together, let not man put asunder.
The question as to the right way of believing is that which puzzles many and engrosses all their anxiety to the exclusion of far greater questions as to the work of him who is the object of their believing. Thus their thoughts run in a self-righteous direction and are occupied, not with what Christ has done but with what they have yet to do to get themselves connected with his work. (pp. 12-13, The Everlasting Righteousness)There are as many struggles with faith as there are Christians... and yet one thing remains true: as Bonar writes earlier, "That which satisfies the holiness of God cannot but satisfy the conscience of the sinner. God, pointing to the altar, says 'That is enough for me'; the sinner responds and says, 'It is enough for me.'" (pp. 12, The Everlasting Righteousness) Christ's life and death - his righteousness - satisfies for the believer's sins, and propitiates them. Completely. It is finished, as Christ said, dying, on the cross.
Let us NEVER think that anything in our believing qualifies us - or any wavering of our trust in difficult times DISqualifies us. We contribute zip. We are His because HE made us His, drew us with irresistible grace, and accounts us His children. We are His because HE gave us faith - led us to trust Him for all - and, because we remain frail men and women, we sometimes struggle to keep the constant strength of faith that we sometimes exhibit. Christ has died, Paul said. Christ is risen. Christ shall come again.