The following is by Rev. Prof. Johann Gerhard (October 17, 1582 – August 17, 1637), orthodox Lutheran Church Father and the quote is from The Treasury of Daily Prayer (CPH):
If someone wants to describe adequately the usefulness of pious, earnest prayer, he will, in my opinion, surely find a beginning more easily than a conclusion. Pious prayer offered in faith is familiar conversation with God. It is a salutary remedy to all the difficulties of life. It is the key to heaven and the door to paradise. It shows us how much we depend on God, and it is a ladder of ascension to God. it is a shield for our defense and a faithful messenger of the ambassador. It is refreshment in the heat of misfortune; it is medicine during illness. It is a winch, drawing us to heaven, and a vessel that draws water from the font of divine kindness. It is a sword against the devil and a defense against misfortune. It is a wind that blows away evil and brings earthly benefits. It is a nurse that nurtures virtues and conquers faults. It is a great fortification for the soul and gives free access to God. It is a spiritual feast and a heavenly delicacy. It is a consolation for the dejected and a delight for the holy. It grants knowledge of the secret things of God and acquires His gifts. It upholds the world and rescues people. It is a joy for the heart and a jubilation for the mind. It follows God’s gift of grace, and it leads ahead into glory. It is a garden of happiness and a tree full of delights. It calms the conscience and increases our thankfulness. It sends demons running and draws angels close. It is a soothing remedy for the misfortunes of this life and the sweet smell of the sacrifice of thanksgiving. It is a foretaste of the life to come and sweetens the bitterness of death.
Whoever is truly a child of God through faith will, with childlike trust, address his or her heavenly Father every day in prayer. The one in whose heart the Holy Spirit has made His home will, as a spiritual priest, daily offer to God this incense of prayer. There are four immovable truths on which our confidence to pray rests. Because of these, we may be certain that our heavenly Father mercifully hears our prayers. The truths on which our certainty rests are:
(1) God’s omnipotent kindness;
(2) God’s unfailing truthfulness;
(3) Christ’s intercession as our mediator; and
(4) the Holy Spirit’s testimony.