There is no contradiction between Jesus' and Paul's instructions regarding our practice of prayer. It is only by misunderstanding one or both of their statements that one might perceive an apparent conflict. Jesus said:
"And when you are praying, do not use meaningless repetition as the Gentiles do, for they suppose that they will be heard for their many words. So do not be like them; for your Father knows what you need before you ask Him," (Matthew 6:7-8).
"Rejoice always; pray without ceasing; in everything give thanks; for this is God’s will for you in Christ Jesus," (1 Thessalonians 5:16-18).
If we understand Jesus to simply mean we should keep our prayers short and if we understand Paul to mean that I should be speaking words to God literally every second of every day, we certainly have a problem. Endless recitation and prayers of only a few words cannot be reconciled. However, this sort of shallow misreading of the text is not at all what the authors' really had in mind. Note that Paul elsewhere wrote to the Romans that they should be:
"rejoicing in hope, persevering in tribulation, devoted to prayer," (Romans 12:12).
To the Colossians that they should:
"Devote yourselves to prayer, keeping alert in it with an attitude of thanksgiving," (Colossians 4:2).
And to the Philippians:
"Rejoice in the Lord always; again I will say, rejoice! Let your gentle spirit be known to all men. The Lord is near. Be anxious for nothing, but in everything by prayer and supplication with thanksgiving let your requests be made known to God," (Philippians 4:4-6).
Paul was not saying that our entire life would be one endless prayer, talking on and on forever. He was advising a devotion to the practice of regular prayer and a perpetual attitude of thanksgiving. He meant that we should never tire of going to God in prayer, expressing our thanks for all things, making our petitions known, and leaning on Him. As James also writes:
"Is anyone among you suffering? Then he must pray. Is anyone cheerful? He is to sing praises," (James 5:13).
Every kind of situation should draw us back to prayer. Christians ought to be a people defined by prayer. Indeed, of the very first church in the book of Acts it is said:
"They were continually devoting themselves to the apostles’ teaching and to fellowship, to the breaking of bread and to prayer," (Acts 2:42).
Such prayer, as Jesus warned, should not be a lengthy, self aggrandizing ritual or an attempt to manipulate God into doing what we want through the number or eloquence of our words. As Solomon wrote:
"Do not be hasty in word or impulsive in thought to bring up a matter in the presence of God. For God is in heaven and you are on the earth; therefore let your words be few. For the dream comes through much effort and the voice of a fool through many words," (Ecclesiastes 5:2-3).
He went on to elaborate:
"For in many dreams and in many words there is emptiness. Rather, fear God," (Ecclesiastes 5:7).
And warned again in the Proverbs:
"When there are many words, transgression is unavoidable, But he who restrains his lips is wise," (Proverbs 10:19).
Job, likewise, at the end of his trials, learned and declared:
"Behold, I am insignificant; what can I reply to You? I lay my hand on my mouth. Once I have spoken, and I will not answer; Even twice, and I will add nothing more," (Job 40:4-5).
We ought not to be impetuous or flippant when we approach the Lord of the Universe. He is holy. We also ought not to be long winded as if we really have to explain things to God. We ought to remember to whom we are speaking, God knows all things, even the things we are praying about. We pray because it pleases God, not because he needs to be reminded of or instructed on anything:
"Who has directed the Spirit of the Lord, Or as His counselor has informed Him? With whom did He consult and who gave Him understanding? And who taught Him in the path of justice and taught Him knowledge And informed Him of the way of understanding?' (Isaiah 40:13-14).
So Jesus instructs that when we pray, we ought to be concise, reverent, mindful of who God is and magnifying God rather than ourselves; seeking His kingdom and not our own desires. Our words, therefore, on any given subject about which we pray will generally be few. Still, Jesus says "when you are praying." His instructions assume a regular practice of prayer. Therefore, far from contradicting Paul's admonition, Jesus' words are complementary to Paul's.
Inside the Bible
Matthew 6:5–8, "When you pray, you are not to be like the hypocrites; for they love to stand and pray in the synagogues and on the street corners so that they may be seen by men. Truly I say to you, they have their reward in full. But you, when you pray, go into your inner room, close your door and pray to your Father who is in secret, and your Father who sees what is done in secret will reward you. And when you are praying, do not use meaningless repetition as the Gentiles do, for they suppose that they will be heard for their many words. So do not be like them; for your Father knows what you need before you ask Him."
1 Timothy 2:1, "First of all, then, I urge that entreaties and prayers, petitions and thanksgivings, be made on behalf of all men."
The Psalms say:
Psalm 5:3-4, "In the morning, O Lord, You will hear my voice; In the morning I will order my prayer to You and eagerly watch. For You are not a God who takes pleasure in wickedness; No evil dwells with You."
Jude 20–21, "But you, beloved, building yourselves up on your most holy faith, praying in the Holy Spirit, keep yourselves in the love of God, waiting anxiously for the mercy of our Lord Jesus Christ to eternal life."
What is prayer?
Prayer is the practice of the presence of God. It is the place where pride is abandoned, hope is lifted, and supplication is made. Prayer is the place of admitting our need, of adopting humility, and claiming dependence upon God. Prayer is the needful practice of the Christian. Prayer is the exercise of faith and hope. Prayer is the privilege of touching the heart of the Father through the Son of God, Jesus our Lord.
Can we change God's mind with prayer?
We should pray because God tells us to pray and because we are told such prayers matter to God. We should pray because it glorifies God. We should pray so that we can participate in spiritual battle, be humbled. Finally, we should pray because it changes us.
"In the Bible prayer is worship that includes all the attitudes of the human spirit in its approach to God. The Christian worships God when he adores, confesses, praises and supplicates him in prayer. This highest activity of which the human spirit is capable may also be thought of as communion with God, so long as due emphasis is laid upon p 948 divine initiative. A man prays because God has already touched his spirit. Prayer in the Bible is not a ‘natural response’ (see Jn. 4:24). ‘That which is born of the flesh is flesh.’ Consequently, the Lord does not ‘hear’ every prayer (Is. 1:15; 29:13). The biblical doctrine of prayer emphasizes the character of God, the necessity of a man’s being in saving or covenant relation with him, and his entering fully into all the privileges and obligations of that relation with God." (Wood, D. R. W., and I. Howard Marshall. New Bible Dictionary. Leicester, England; Downers Grove, IL: InterVarsity Press, 1996.)