What did prayer look like in the Old Testament?
There are many examples of prayer in the OT, by people all across the social spectrum (think: Daniel was thrown in the den of lions because of his daily prayer discipline; Esther appeals to all the Jews in Persia to fast and—by implication—pray; Jonah, David, Hannah, etc...). So we see both individual and corporate prayers in the OT…and we see God answering those prayers! So it is safe to say that all people could go to God in prayer…and He would hear those prayers and respond according to His own good will. But a person's relationship with God under the Old Covenant was not the same as in the New Covenant. Because the OT believer had to continually offer sacrifices for sins…he/she was perpetually unable to “draw near to God” because of sin. Consider these passages:
(Proverbs 28:9) 9 If one turns away his ear from hearing the law, even his prayer is an abomination.
(Isaiah 59:2) 2 but your iniquities have made a separation between you and your God, and your sins have hidden his face from you so that he does not hear.
(John 9:31) 31 We know that God does not listen to sinners, but if anyone is a worshiper of God and does his will, God listens to him.
[NOTE: Of course, we are all sinners (OT and NT)…but in the New Covenant, our relationship with God can be PERMANENTLY restored! We can be considered PERFECT before God, because of the cross (Hebrews 10:14).]
But under the New Covenant, the believer's relationship to the Trinity has changed. We are now adopted children of the Father, co-heirs with the Son, sealed and empowered by the Holy Spirit.
Look at what Jesus says to His disciples about what they should expect concerning prayer AFTER the ascension and the giving of the Holy Spirit...
(John 16:23–24) "In that day you will ask nothing of me. Truly, truly, I say to you, whatever you ask of the Father in my name, he will give it to you. 24 Until now you have asked nothing in my name. Ask, and you will receive, that your joy may be full."
Jesus explains that asking for something “in His Name” is more significant, because that request will certainly be fulfilled (“...he will give it to you…”). I don’t think we can say that about any OT prayer. Now that the Son is seated at the right hand of the Father, interceding for us (Romans 8:34), our prayers ("in Jesus’ name") are certain to be heard by the Father, through the Son…and will be answered. No longer must we wonder, “Am I in right relationship with God today, that I may go to Him in prayer, and KNOW that He is hearing, and will respond?”