Prayer must be of the heart. It is not learning a formula nor simply repeating words. Prayer is never intended to be a vain repetition of memorized words. Jesus gave us a model prayer to understand our personal relationship with God our Father. If we only say memorized words, without considering their meaning, our heart is not in it. Prayer must come from the heart before it is spoken from the lips.
Prayer of the heart is relational. It is fellowship with God. The believer who prays effectively has a personal relationship with God as Father. Through faith in Jesus Christ, our hearts are open to God as our heavenly Father. Jesus said, "In this manner, therefore pray, our Father in heaven, hallowed be your name"(Matthew 6:9). A personal relationship with God as our Father is the heart of prayer.
All personal relationships are heart to heart. So it is with God our Father and prayer from the heart. God's Spirit works in our hearts. Our prayers become more than words from our lips. Our hearts are open to Him.
We share the thoughts, desires, and emotions of our hearts with God in prayer. He speaks to our hearts. "Today, if you will hear His voice, do not harden your hearts as in the rebellion" (Hebrews 3:15). He often brings Bible verses to our attention. He may bring people to mind, because we need to pray for them.