“If anyone would come after me, he must deny himself and take up his cross and follow me. For whoever wants to save his life will lose it, but whoever loses his life for me and for the gospel will save it.
What good is it for a man to gain the whole world, yet forfeit his soul? Or what can a man give in exchange for his soul?
If anyone is ashamed of me and my words in this adulterous and sinful generation, the Son of Man will be ashamed of him when he comes in his Father’s glory with the holy angels.” Mk 8:34-38

Here, Jesus speaks of the costs associated with following him.

1. Those who follow Christ will seek to put Christ first, rather than themselves. It is expressed in one of the earliest and most fundamental Christian confessions – Jesus is Lord. It is important to note that self-denial doesn’t make one a Christian, but being a Christian causes one to be self-denying.
   Just as litmus paper turning red doesn’t make a substance acidic, but shows the acidity of a substance, self-denial is the mark of a Christian.
   This is not to say Christians will be perfect, for there are still remnants of the old sinful, self-seeking, nature, but someone who claims to be a Christian but shows no hint of self-denial needs to examine themselves.
   It is also important to note that this self-denial is not the same as being joyless or ascetic, for if we put Christ first, his joy will be in us and will be made complete.
2. This self-denial won’t be easy – Jesus likens it to taking up a cross. For although our salvation is given freely, it will be cost us many things in this world. It requires a constant battle against the old, self-seeking, nature. It may cost us many things which this world counts as worthy, such as money and status. However, we can take none of this with us when we die.
3. Despite how much personal cost is involved, a look at the bigger picture brings it all into perspective. When you consider how much this cost God, who gave his Son as a ransom for many. When you consider that Jesus Christ laid down his life of his own accord for his people.
   

   Were the whole realm of nature mine,
   That were a present far too small;
   Love so amazing, so divine,
   Demands my soul, my life, my all. Isaac Watts

As Paul writes to the church in Romans, he shows that salvation by grace alone, through faith alone, in Christ alone, is found in the pages of the Old Testament. Going back to Genesis and Abraham.

Abram believed the LORD, and he credited it to him as righteousness. Gen 15:6

For Abraham’s actions were all marks of his faith. Abraham did not earn his righteousness through works, but through God crediting it to him.
Similarly, David says in the Psalms.

Blessed is he whose transgressions are forgiven, whose sins are covered.
Blessed is the man whose sin the LORD does not count against him and in whose spirit is no deceit. Ps 32:1,2

Even though what we now know in the New Testament was not so clear in the Old, there was enough there for people to trust in the Messiah who was to come – the Lord Jesus Christ. Jesus Christ came to save his people – throughout all time. If the people of the Old Testament could trust in the Lord for their salvation, through their limited knowledge, how much more should we trust in the Lord with the knowledge we possess.