47 What is the hope of which this futile passion of yours is in pursuit? What is the assurance of your salvation which emboldens you with blasphemous licence of tongue to maintain that Christ is a creature, and not a Son? It was your duty to know this mystery, from the Gospels, and to hold the knowledge fast. For though the Lord can do all things, yet He resolved that every one who prays for His effectual help must earn it by a true confession of Himself. Not, indeed, that the suppliant's confession could augment the power of Him, Who is the Power of God; but the earning was to be the reward of faith.
So, when He asked Martha, who was entreating Him for Lazarus, whether she believed that they who had believed in Him should not die eternally, her answer expressed the trust of her soul— Yea, Lord, I believe that You are the Christ, the Son of God, Who has come into this world. This confession is eternal life; this faith has immortality. Martha, praying for her brother's life, was asked whether she believed this. She did so believe. What life does the denier expect, from whom does he hope to receive it, when this belief, and this only, is eternal life? For great is the mystery of this faith, and perfect the blessedness which is the fruit of this confession.
Source: On the Trinity (New Advent)