Mental or interior prayer (as distinct from vocal prayer, which normally is expressed by set formulas) begins with a serious consideration of the truths of the Faith, with a view to their practical application to our daily lives. During mental prayer, you grow in the knowledge of your faith and acquire principles of right living by applying yourself to prolonged reflection on some doctrine of the Church or teaching of our Savior.
This reflection, however, is only a point of departure for spontaneous prayer. Such prayer is the principal part of every meditation. The subject of meditation provides incentives and matter for prayer. Prayer really begins at the moment when your will, set on fire with love, enters into direct communication with God and yields lovingly to Him in order to please Him and to fulfill His commands and desires. Therefore, the essential element in mental prayer is contact with God, the source of all holiness.
It is in the heart that prayer essentially dwells. The Blessed Virgin Mary kept the words of Jesus “in her heart” (cf. Luke 2:51). When our Lord taught His Apostles to pray, He did not bid them to apply themselves to an intellectual exercise, but to express the love of their hearts as simply as do little children. If meditation does not lead to conversation with God, then it is no longer an internal prayer, but a kind of study or examination of conscience, or a period of spiritual reading for information and enjoyment. The important point is not to think much, but to love much.
Successful meditation depends above all on prayer and a longing for perfection. Use a book only as a means of uniting your heart with God, keeping in mind that Sacred Scripture is the best source. If you can commune with God only through vocal prayer, then continue vocal prayer. The Holy Spirit has many ways of leading a soul nearer to God. If you are able to speak habitually and simply with God, and can gain much spiritual food from this communication, do not tie yourself down to any particular method. Once the best way is found, keep faithfully to it until the Holy Spirit draws you to another way. Be generously docile to His grace. Try to make at least a short meditation every day. You will find this practice an important means of growing in the love of God. You will gradually acquire the spirit of prayer, which, in other words, means the habit of having recourse to God more and more frequently.
The difference between vocal prayer and interior prayer is this: in vocal prayer we use a prepared form of words, either a standard prayer from a prayer book, or a prayer we have made up by ourselves, and we recite this prayer, aloud or silently, from the book or from memory. In interior prayer, we do not use a prepared form of words at all, but merely raise our minds and hearts to God spontaneously, addressing Him with words of love or in no words at all.
Vocal prayer also forms an essential prayer of the external worship of God. Vocal prayer, expressed in public worship, has special power with God and is very pleasing to Him, for Our Lord said, “Again I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in Heaven. For where two or three are gathered in my name, there am I in the midst of them” (Matthew 18:19–20).
“Pray then like this: Our Father who art in Heaven, Hallowed be thy name. Thy kingdom come, Thy will be done, On earth as it is in Heaven” (Matthew 6:9–10).
There comes a point in the spiritual life when, after faithful practice of Christian virtue and prayer, the Holy Spirit acts and prays within us. As Scripture says, “Likewise the Spirit helps us in our weakness; for we do not know how to pray as we ought, but the Spirit himself intercedes for us with sighs too deep for words” (Romans 8:26).
Through the Gifts of the Holy Spirit, we come to experience what is called infused prayer. Infused prayer means “poured into.” “God’s love has been poured into our hearts through the Holy Spirit who has been given to us” (Romans 5:5).
The grace of infused prayer does not require any extraordinary mystical graces or experiences. It is the development of the ordinary graces of Baptism.