Why is prayer necessary for Christians?

Because it is the chief part of gratitude which God requires of us, and because God will give His grace and Holy Spirit only to those who sincerely beseech Him in prayer without ceasing, and  who thank Him for these gifts. 

What is contained in the prayer which pleases God and is heard by Him?

First, that we sincerely call upon the one true God,  who has revealed Himself to us in His word, for all that He has commanded us to ask of Him.  Then, that we thoroughly acknowledge our need and evil condition so that we may humble ourselves in the presence of His majesty.  Third, that we rest assured that, in spite of our unworthiness, He will certainly hear our prayer for the sake of Christ our Lord, as He as promised us in His word. 

What has God commanded us to ask of Him?

All things necessary for soul and body which Christ the Lord has included in the prayer which He Himself taught us. 

I. God Directs His People to Pray

A. The God of grace directs the redeemed to pray prayers of thanksgiving. Offer unto God thanksgiving; and pay thy vows unto the most High:  (Psalm 50:14)

B. God assures believers that prayer is worth the undertaking.  Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you: For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened.  (Matthew 7:7-8)

C. Prayer and its answer illustrate the fundamental features of grace and Providence.  And I [Jesus] say unto you, Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you. For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened. If a son shall ask bread of any of you that is a father, will he give him a stone? or if he ask a fish, will he for a fish give him a serpent? Or if he shall ask an egg, will he offer him a scorpion? If ye then, being evil, know how to give good gifts unto your children: how much more shall your heavenly Father give the Holy Spirit to them that ask him?  (Luke 11:9-13)

II. God Listens for Certain Features in Prayer

A. God expects honesty.  The LORD is nigh unto all them that call upon him, to all that call upon him in truth.  (Psalm 145:18)  God is a Spirit: and they that worship him must worship him in spirit and in truth.  (John 4:24)

B. Proper prayer conforms to God=s revelation in Scripture.  And this is the confidence that we have in him, that, if we ask any thing according to his will, he heareth us:  (1 John 5:14)

C. God will respect the humility of a believer who turns for help without understand everything he needs to seek.  If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him.  (James 1:5)

D. Appropriate prayer will reflect genuine humility and respect for God=s revealed will.  For all those things hath mine hand made, and all those things have been, saith the LORD: but to this man will I look, even to him that is poor and of a contrite spirit, and trembleth at my word.  (Isaiah 66:2)

E. God likes to have His people express entire confidence in Him.  O our God, wilt thou not judge them? for we have no might against this great company that cometh against us; neither know we what to do: but our eyes are upon thee.  (2 Chronicles 20:12)  

F. Genuine humility turns to God and from rebellion at the same time.  If my people, which are called by my name, shall humble themselves, and pray, and seek my face, and turn from their wicked ways; then will I hear from heaven, and will forgive their sin, and will heal their land.  (2 Chronicles 7:14)

G. Do not expect to waver and prosper.  But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed.  (James 1:6)

H. Always remember that the electing love of God and the substitutionary atonement are part of any successful appeal to God.  And whatsoever ye shall ask in my name, that will I do, that the Father may be glorified in the Son. If ye shall ask any thing in my name, I will do it.  (John 14:13-14)

III. God=s Gracious Providence Sets the Parameters of Prayer