The Lord will save all who believe the good news of the Gospel, but to those who refuse, there is a destruction awaiting them in eternity. You see, Jesus died for our sins, but to refuse His free gift of salvation is to suffer in a Christ-less place called Hell.
 My mouth shall speak the praise of the LORD: and let all flesh bless His holy name for ever and ever.
The entirety of this Psalm is about reasons why we should praise God. David concludes this Psalm by saying in effect he will continue to praise God as long as he lives and let everyone else praise Him as long as people and creatures ("flesh") exist. How do "creatures" praise God? It is not verbally. Think about it. They have have food to eat and places to live so in effect they are praising God because they have that benefit for their lives.
Let's summarize this chapter of how we use our lives to make a difference for Him. First, it requires we acknowledge His existence and the fact He knows all things, Next, we must be willing to submit our lives to Jesus for the full forgiveness of our sins. Then it requires us to praise Him for the blessings He has bestowed on our lives. After that, it is about the realization that the results of our lives in effect are "His problem" and we don't have to worry about our future. Finally, we go forward making the best decisions possible, living by biblical principals and trusting in His guidance for our lives. Now that is why we praise Him.