Q. 116. Why is prayer necessary for Christians?

A. Because it is the chief part of thankfulness which God requires of us: and also, because God will give his grace and Holy Spirit to those only, who with sincere desires continually ask them of him, and are thankful for them.

Q. 117. What are the requisites of that prayer, which is acceptable to God, and which he will hear?

A. First, that we from the heart pray to the one true God only, who has manifested himself in his word, for all things, he has commanded us to ask of him; secondly, that we rightly and thoroughly know our need and misery, that so we may deeply humble ourselves in the presence of his divine majesty; thirdly, that we be fully persuaded
that he, notwithstanding that we are unworthy of it, will, for the sake of Christ our Lord, certainly hear our prayer, as he has promised us in his word.

Q. 118. What has God commanded us to ask of him?

A. All things necessary for soul and body; which Christ our Lord has comprised in that prayer he himself has taught us.

Q. 119. What are the words of that prayer?

A. Our Father which art in heaven,
1. Hallowed be thy name.
2 Thy kingdom come.
3 Thy will be done
on earth, as it is in heaven.
4 Give us this day our daily bread.
5 And forgive us our debts,
as we forgive our debtors.
6 And lead us not into temptation,
but deliver us from evil.
For thine is the kingdom, and the power, and the glory, for ever. Amen.

http://www.ccel.org/creeds/heidelberg-cat-ext.txt