Essential Gospel

“This is the time of fulfillment. The kingdom of God is at hand. Repent, and believe in the gospel.”  Jesus' first words in the Gospel of Mark (Mk 1:15)
Before considering the most fundamental part of the gospel, we must first consider just how great God's call is for each of us. Here's what the Church teaches:
"All Christians in any state or walk of life are called to the fullness of Christian life and to the perfection of charity." All are called to holiness: "Be perfect, as your heavenly Father is perfect" ... (see Catechism #2012-16)
God calls us to be holy, to be free from all sin and attachment to sin. He wants to heal us from the fallen nature we inherited from Adam & Eve. He wants to elevate us to be sharers in his own divine life. In the end, he wants to make us saints (see Catechism #1691-96). In fact, we will not enter heaven until our lives have been perfected in this way (see Rev 21:27, also see Life Everlasting)

So how is this tremendous call to holiness achieved? It can't be achieved without the help of God's infinite grace. No matter how hard we work, no matter how many good-deeds we do, we would still fall short. We need so much more than our mere efforts, we need the sacrifice of Christ.

image source
Christ's death on the cross is the perfect sacrifice made by God for us. It makes our call to holiness possible. Without it, we cannot stand before God.When Christ rose from the dead, a crown was placed on this infinite work of love, proving its worth for ever. 

Let's consider the words of St. Paul with regards to this essential gospel of Jesus Christ:
"Now I am reminding you, brothers, of the gospel I preached to you, which you indeed received and in which you also stand. Through it you are also being saved, if you hold fast to the word I preached to you, unless you believed in vain. For I handed on to you as of first importance what I also received: that Christ died for our sins in accordance with the scriptures; that he was buried; that he was raised on the third day in accordance with the scriptures; that he appeared to Cephas, then to the Twelve. After that, he appeared to more than five hundred brothers at once, most of whom are still living, though some have fallen asleep. After that he appeared to James, then to all the apostles." (1 Cor 15: 1-7)
Placing our faith in this essential gospel is what transforms us into the people God made us to be, into the holiness that God requires from each of us. 

The death and resurrection of Jesus establishes a new covenant between God and his people. This basically means that God will be faithful in saving us if we put our faith in him and cooperate with his grace. We enter into this covenant in Baptism

Keep in mind, though, that as wonderful and awesome Baptism is, because it brings us into a covenant relationship with God almighty, it is only thepromise of our salvation, not the ultimate fulfillment of it. Again, we still need to cooperate with God over the course of our entire lives. In other words, through his covenant, God abundantly provides each one of us with all that we need for salvation in Baptism (Grace and the Holy Spirit), but we must freely choose to accept and use these gifts. St Augustine said it best:

"Without God I can't, but with out me God won't" 
St. Augustine

Unfortunately, many are so preoccupied with lesser important things that they fail to truly embrace the covenant. If they persist in this indifference, what will their eternal outcome be?

Jesus revealed to St. Faustina: "My Heart overflows with great mercy for souls (...). If only they could understand that I am the best of Fathers to them and that it is for them that the Blood and Water flowed from My Heart as from a fount overflowing with mercy. For them I dwell in the tabernacle as King of Mercy. I desire to bestow My graces upon souls, but they do not want to accept them (...). Oh, how indifferent are souls to so much goodness, to so many proofs of love! (...). They have time for everything, but they have no time to come to Me for graces." (Diary #367)

Here are some other excerpts from St. Faustina's diary that are worth considering. They're very eye-opening in a world that has stopped thinking about stuff like this.

The bottom line, take the sacrifice of Christ seriously. It is our merciful God offering his children the opportunity for eternal happiness. Don't let the things of this world detour you from fully desiring and embracing this gift. Here is the Chaplet of Divine Mercy, a prayer that will help us stay focused.