I'm looking over the passage in Job 22:21-28, I saw verse 21 which struck me in reading a couple of days ago. So much so that I actually remembered the exact chapter and verse correctly today. Remember that Job is considered by many to be the oldest book written for the Bible. Yes, the beginning of Genesis recounts history much older, but it is also considered as having been written down by Moses from older writings, with Moses serving more as editor than author. In any case, even though Job doesn't mention the Law, it does bring up some very "advanced" theology for such a "primitive" knowledge of God.
The setting is Eliphaz the Temanite's speech to Job starting in verse 2. Eliphaz is accusing Job of numerous things, apparently by "reading" the "fruit" of Job's life by the misery Job is in now. So, Eliphaz begins by pointing out that we have nothing to give to God that is not already His by right, says Job was an unjust magistrate among other things, accuses Job of thinking God cannot see his sins from way up in heaven (perhaps implying Job is an atheist) and finally references the "old world" from before the Flood. Which brings us to the passage in question.
Acquaint now thyself with him, and be at peace: thereby good shall come unto thee. Receive, I pray thee, the law from his mouth, and lay up his words in thine heart.
The "him" here is God and the suggestions from Eliphaz are those good for any Christian to remember. I believe that there were numerous writings and accounts of God and His interactions with people even before Moses. Even in the Scriptures we have mention of books that we no longer have any record of and wouldn't know about if they weren't mentioned or quoted in Scripture. Eliphaz calls on Job (and by extention, all people) to aquaint themselves with God and be at peace towards God and His will (i.e. not disobedient). Specifically, receive the law from His mouth and lay up His words in your heart, so you will know obedience. By this good (blessings) will come to you.
If thou return to the Almighty, thou shalt be built up, thou shalt put away iniquity far from thy tabernacles. Then shalt thou lay up gold as dust, and the gold of Ophir as the stones of the brooks. Yea, the Almighty shall be thy defence, and thou shalt have plenty of silver.
Eliphaz is calling for Job to repent and return to God. God will bless him again with worldly goods to go with that peace. The Almighty will be his defence when you trust Him.
For then shalt thou have thy delight in the Almighty, and shalt lift up thy face unto God. Thou shalt make thy prayer unto him, and he shall hear thee, and thou shalt pay thy vows. Thou shalt also decree a thing, and it shall be established unto thee: and the light shall shine upon thy ways. (Job 22:26-28)
Looking always to God, He will hear your prayers because you will be one of His children, as is promised in Scripture. Paying your vows refers to one sense of taking the Lord's Name in vain and bearing false witness and is referred to in the New Testament when told not to swear to the creation (as the Jews often did to avoid taking His Name in vain). Decreeing a thing and having it happen refers to binding and loosing and being able to ask anything and God give it to you, even to casting a mountain into the sea. This was not something new with Jesus, but was well known in Jewish thought with the provision that anyone who was that much in line with God would also never ask anything outside of God's will. Light shining on all your ways is a generic blessing where God will show you the path and direct your steps in the best way. All things work together for good for those that trust in the Lord.