Case 1. What is to be done if a man, after receiving, still doubts whether he has faith or not? He must strive against doubting, and endeavor to believe, being heartily sorry for the weakness and infirmity of his faith. And let him withal consider and remember that God has not only given His promise, but set apart this sacrament to be a special sign and pledge of His mercy contained in the promise for the upholding and strengthening of man's faith.
But some man will say, My endeavor is nothing, if doubting prevails. Answer. It is not so. For if a man is heartily sorry for his infirmity, if he strives to believe, if in heart he hungers and thirsts after Christ, faith is begun, and he in some sort does apprehend Christ. The poor beggar by the highway side enjoys the alms that is given him, though he receives it with a lame and leprous hand. The stomach that loathes physick, if it receives into it at the first but one will be able at length to take benefit by a greater quantity, and in the meantime it receives good. The man who is in closed prison, if he sees but one little bean of the sun by a small crevice, b that very beam he has use of the sun, though he sees not the whole body of the sun. In like manner, though our faith, the hand of our soul, is mingled with weakness and corruption, though we feel ever so little measure of God's grace in us, yea though our knowledge is ever so small, yet it is an argument that the Spirit of God begins to work in our hearts, and that we have by God's mercy begun to lay hold on Christ" pg. 298, 299 William Perkins 'The Works of William Perkins" Vol. 8