The little word "so" is amazing. It can connect what was previously said to a logical response when used in the sense of "therefore." God loved you , SO you should love each other. Breaking the law has serious consequences, SO wear your seat belt and drive the speed limit. You get the picture. It is a bridge in logic.
"So" can also be used as sarcasm. When someone makes a statement, another might say, "So?" In this case, the one responding is declaring that he does not recognize the bridge in logic. He might as well have said, "What's your point?" In other words, the statement is either incomplete, or it does not carry the weight of evidence which demands an obvious response. This use of the term is frequently used by teens who want to challenge authority. It is also used by those who want to reject the shared Word of God, or who want to deny the significance of a Christian's testimony. It is a way of saying, "I don't care."
In Psalm 107:2, the word is used almost as a command! "Let the redeemed of the LORD say so...." Here, the Spirit uses the word to instruct believers to declare what the LORD has done for them. It is demanding that believers tell others "This is so." It defines one's current situation based upon the events that led up to the present. It is what is true today. It is our testimony.
Verses eight, fifteen, twenty-one, and thirty-one of Psalm 107 carry the message throughout the psalm. The writer repeats the exact same words over and over again: "Oh that men would praise the LORD for His goodness, and for His wonderful works to the children of men!" It is as though the writer is begging, rather than demanding. You can almost hear him crying out to us, "Please, I beg of you. Declare for all to hear, what God has done for you!"
Of course, if you are unaware of the Lord's existence, or of His presence in your everyday life, you have nothing to say. If you do not believe God loved you and expressed it in the sacrifice of His Son on your behalf, you have nothing to say. If you are living a life that does not line up with what the Word says a believer's life should be, then for God's sake, please have nothing to say! Yes, the redeemed of the Lord are to testify, but their life should be without hypocrisy (Jam. 3:17). Paul put it this way, "...Let every one that nameth the name of Christ depart from iniquity" (2 Tim. 2:19). In other words, "Get your act together, and then be a witness for Jesus."
Does that mean one has to be perfect in order to testify? No! It means that a believer is to have repented and to have confessed his sins before witnessing for Christ. 1 John 1:9 teaches us that when we do this, we are returned to a state of righteousness. The result, or fruit of righteous produces life, even to the point of winning souls to Christ (Prov. 11:30). It is His righteousness poured out through us that quenches those who are thirsty (Matt. 5:6). Be slow to speak, but when you do, say SO!