Deuteronomy. The majority of this book is devoted to the words of Moses after 40 years of wandering in the desert. The Israelites were on the plains of Moab, poised to enter the promised land, and so this was Moses' final opportunity to address them before his death. There are many great passages in this Deuteronomy, so much so, that it was quoted by Christ more times than any other book in the Old Testament. Perhaps the most important passage in this book is found in Chapter 6 where Moses instructs the Israelites to "Love the LORD your God with all your heart and with all your soul and with all your strength." (Deuteronomy 6:5, NIV, cf. Matthew 22:36-40)
Joshua. This book picks up where Deuteronomy leaves off, following Moses' death. Under the leadership of Joshua, the Israelites conquer Canaan and enter the promised land. The highlights of Joshua include the fall of Jericho (6:1-27) and Joshua's farewell addresses (23:1-24:28). The latter passage contains what is arguably Joshua's most memorable quote: "But if serving the LORD seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your forefathers served beyond the River, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the LORD." (Joshua 24:15)
Judges. This book devotes much of its attention to the "lawless" period of the Israelites time in the promised land prior to the establishment of Saul as their first King. During this period, God showed his mercy by providing "judges" to rescue Israel at various times, following the death of Joshua. These were not judges in the sense that we might think of them today, but rather, were military leaders who were called to lead Israel in battles against their enemies. The irony of Judges is in the sin of the people--sin that led to defeat at the hands of their enemies in the first place. Indeed, had the people not turned away from God there would have been no defeat, and hence, no need for a judge to rescue them. Thirteen judges are described in this book, perhaps the most well-known of which is Gideon, described in chapters 6-8; and Sampson, described in chapters 13-16.
Ruth. This four chapter book tells the story of Ruth, a Moabite woman of great character who travels with her mother-in-law to Bethlehem following the deaths of their husbands. It is there that Ruth meets Boaz, whom she later marries. Though Ruth is a foreigner, she and Boaz would be included in the genealogy of Christ (Matthew 1:5). But this is not the only connection that Ruth and Boaz have to Christ. Indeed, the whole story of how Ruth, a foreigner, is redeemed by Boaz creates a striking image of how Christ will likewise redeem all of humanity through his death on the cross. In Ruth we see in the way in which God works to redeem an individual; through Christ, we see the way in which God works to redeem humanity.
In order to answer this question, we must first ask an even more basic question, namely, how were people in Old Testament days were saved prior to the time of Christ?
Paul addresses this question in his epistle to the Roman church. In his letter, Paul's wanted to emphasize to the Roman believers that salvation (i.e., justification) was theirs through faith in Christ and not through any works of righteousness. To illustrate this, Paul drew on the example of Abraham to show that salvation has never been the result of works of righteousness, even in Old Testament times! In OT times, God declared people righteous on the basis of their faith (i.e., belief) in Him, just as today he now declared people righteous on the basis of their faith in His son.
Paul quotes Genesis in Romans 4, reminding his readers that "Abraham believed the LORD, and he credited it to him as righteousness." (Romans 4:3, cf. Genesis 15:6). These OT believers performed sacrifices to God, looking forward to the day of the coming Messiah who would usher in a new covenant (Jeremiah 31:31-34, Ezekiel 36:24-28). So we see that individuals in the OT were saved by faith in the same way that people in NT times were.
In light of this, we return to the question about those who didn't make it onto Noah's ark. In Genesis 6 we see that the earth had become exceedingly wicked with the exception of Noah (vv.8-9). God was forced to extend His righteous judgement upon the earth because of its wickedness and violence. Those who believed in God were saved--not just spiritually but also from the flood. Those who refused to believe were condemned, as their sins deserved. Throughout the entire process of building the ark (which may have taken as long as 120 years), the people were keenly aware of what Noah was doing. At any time during this extended period of grace they could have turned to God and escaped judgement. Sadly, they chose to reject God and thus, fell to His righteous judgement.
The parallel to our lives today is striking. God has given each of us a period of grace--our own lifetimes--before we face judgement (Hebrews 9:27-28). God offers us a way of escape--but we must choose it now, before our lives--just like the door of the ark--close for good and God's righteous judgement is poured out. Choose to believe in Christ today and receive His gift of forgiveness and salvation, for "now is the time of God's favor, now is the day of salvation." (2 Corinthians 6:2b)