The only way one can believe in eternal security is to only accept a few happy verses and throw the rest of God's Word in the trash.
This is not true at all. The scriptures are without error, inspired, and speak with one voice. We do not throw out some of God's word in order to make ourselves happy. If your interpretation of the rest of God's word doesn't square with the doctrine of eternal security, then perhaps your interpretation of the other verses is wrong.
You must not ignore or cut out verses about eternal security since these are also part of God's word. In order to gain a deeper understanding of the subject, let's turn to some other verses which help us get a clearer picture.
God's Word contains warnings written to Christians clearly indicating that it's very possible to for Christians to fall away from the Lord and end up going to hell.
Of course. God's word contains warnings. But how should we interpret those warnings? Do such warnings clearly indicate the possibility of falling away? Yes. Do such verses contradict the doctrine of eternal security? Not at all. Although falling away is something that some believers do; it isn't something that renders the promises of God null and void. Contingency doesn't necessarily indicate uncertainty.
Please take a look at the following passage, which comes from a conversation Jesus was having with the Pharisees.
John 8:52
The Jews said to Him, “Now we know that You have a demon. Abraham died, and the prophets also; and You say, ‘If anyone keeps My word, he will never taste of death.’ Surely You are not greater than our father Abraham, who died? The prophets died too; whom do You make Yourself out to be?”
The Scribes and Pharisees asked an important question, which Jesus took seriously. Abraham couldn't keep himself from death, let alone keep others from death, and neither could the prophets keep themselves from death. The Pharisee's objection contains an essential underlying assumption. God favored Abraham and blessed him; God was with the prophets and granted them his favor also. The scriptures clearly teach that God was working through the prophets and yet, not one of them was able to keep himself from death. They all died. What makes Jesus believe that he is any different than the other prophets? Jesus responds,
John 8:54-56
Jesus answered, “If I glorify Myself, My glory is nothing; it is My Father who glorifies Me, of whom you say, ‘He is our God’; and you have not come to know Him, but I know Him; and if I say that I do not know Him, I will be a liar like you, but I do know Him and keep His word. Your father Abraham rejoiced to see My day, and he saw it and was glad.”
it is My Father who glorifies me . . .
In this passage, Jesus communicates two important points: firstly, that he is the son of God, and secondly, that the Father will glorify him. What sets Jesus apart from Abraham and the prophets is that, being the son of God, the Father has made promises to him that were not made to Abraham or the prophets. In case we think Jesus' statement was an arrogant and empty boast, the Father himself will later prove that what Jesus said is true.
John 11:40-44
Jesus *said to [Martha], “Did I not say to you that if you believe, you will see the glory of God?” So they removed the stone. Then Jesus raised His eyes, and said, “Father, I thank You that You have heard Me. I knew that You always hear Me; but because of the people standing around I said it, so that they may believe that You sent Me.” When He had said these things, He cried out with a loud voice, “Lazarus, come forth.” The man who had died came forth, bound hand and foot with wrappings, and his face was wrapped around with a cloth. Jesus *said to them, “Unbind him, and let him go.”
This miracle proves that the Father hears the prayers of Jesus and will raise his followers from the dead at the command of Jesus. At the command of Jesus, Lazarus walked out of the tomb alive. And he will do this again for all of his followers. Jesus' claim to be the resurrection and the live turned out to be true. And he will do it again.
1 Thessalonians 4:16-18
For the Lord Himself will descend from heaven with a shout, with the voice of the archangel and with the trumpet of God, and the dead in Christ will rise first. Then we who are alive and remain will be caught up together with them in the clouds to meet the Lord in the air, and so we shall always be with the Lord. Therefore comfort one another with these words.
with a shout . . .
Paul records the promise of Jesus and how it will take place. The Lord himself will descend from heaven. With a shout (with a loud voice the dead in Christ will will rise.
in Christ
This is the key phrase, spoken often by Paul, essential to our understanding of eternal security. All those "in Christ" will be raised whether dead or alive at the descent of the Lord from heaven.
What are we saying?
The warnings in scripture indicate the contingency associated with salvation. But lest we should mistakenly believe that we are the locus of at the center of the contingency, Jesus and the apostles teach us that God promises to satisfy the contingency himself. Recall from the book of Genesis how God made a covenant with Abraham when God walked between the cuttings alone. This indicated that although God made a covenant with Abraham, which required that Abraham walk in faith and keep his part of the covenant, God would miraculously remove all contingency. The promises associated with the covenant rested on God alone.
We see this clearly in passages such as John 3, where Jesus points out that God is saving those in whom he places his spirit. Paul the apostle picks up on that idea in Romans 8.
Romans 8:9-11
However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him. If Christ is in you, though the body is dead because of sin, yet the spirit is alive because of righteousness. But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ Jesus from the dead will also give life to your mortal bodies through His Spirit who dwells in you.
if indeed the Spirit of God dwells in you . . .
The essential aspect of salvation is whether or not the Spirit of God dwells in you. According to Paul all who have the Spirit of God dwelling in them will be saved. The Spirit himself will give life to your mortal bodies. Today we live in hope of salvation but tomorrow we will raised at the command of Jesus Christ according to a promise of the Father to glorify his son.
Romans 5:1-5
Therefore, having been justified by faith, we have peace with God through our Lord Jesus Christ, through whom also we have obtained our introduction by faith into this grace in which we stand; and we exult in hope of the glory of God. And not only this, but we also exult in our tribulations, knowing that tribulation brings about perseverance; and perseverance, proven character; and proven character, hope; and hope does not disappoint, because the love of God has been poured out within our hearts through the Holy Spirit who was given to us.
Hope does not disappoint . . .
This promise is assured and guaranteed to all those who have persevered under testing. This idea is also found in Jesus' parable of the sower, where he points out that the good soil represents the good and honest heart that will persevere in believing. The Father is the one who prepares the heart. The Father is the one who pours out his spirit into the heart. The Father is the one who puts the Spirit of Christ into those whom he is saving. God removes all contingency.
Eternal security is a miracle of God.