Kenneth,
Regarding your question: "why did Jesus in Mark 16:15 command us to go into all the world and preach the gospel if only the elect could be saved. If we are already predestined, then why command us to spread the gospel. Who is susposed to be saved, since we are predestinated, will be saved."
Witnessing to "all the world" has a twofold purpose. When the gospel is heard by the non-elect, their rejection condemns them. Even the elect must be drawn to Christ, so witnesses are part of the election process. When I first grasped and delt with the election concept, it was difficult. Here is a substantial case for election:
Romans 8:28-30 (ESV)
And we know that for those who love God all things work together for good, for those who are called according to his purpose. For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified.
2 Peter 1:10 (ESV)
Therefore, brothers, be all the more diligent to make your calling and election sure, for if you practice these qualities you will never fall.
1 Peter 1:2 (ESV)
according to the foreknowledge of God the Father, in the sanctification of the Spirit, for obedience to Jesus Christ and for sprinkling with his blood: May grace and peace be multiplied to you.
2 Timothy 1:9 (ESV)
who saved us and called us to a holy calling, not because of our works but because of his own purpose and grace, which he gave us in Christ Jesus before the ages began,
2 Thessalonians 2:13 (ESV)
But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the first fruits to be saved, through sanctification by the Spirit and belief in the truth.
1 Thessalonians 2:12 (ESV)
we exhorted each one of you and encouraged you and charged you to walk in a manner worthy of God, who calls you into his own kingdom and glory.
1 Thessalonians 1:4 (ESV)
For we know, brothers loved by God, that he has chosen you,
Colossians 3:12 (ESV)
Put on then, as God’s chosen ones, holy and beloved, compassion, kindness, humility, meekness, and patience,
Ephesians 2:10 (ESV)
For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them.
Ephesians 1:4-5 (ESV)
even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us for adoption through Jesus Christ, according to the purpose of his will,
Ephesians 1:9-11 (ESV)
making known to us the mystery of his will, according to his purpose, which he set forth in Christ as a plan for the fullness of time, to unite all things in him, things in heaven and things on earth. In him we have obtained an inheritance, having been predestined according to the purpose of him who works all things according to the counsel of his will,
Galatians 1:15 (ESV)
But when he who had set me apart before I was born, and who called me by his grace,
1 Corinthians 1:26-29 (ESV)
For consider your calling, brothers: not many of you were wise according to worldly standards, not many were powerful, not many were of noble birth. But God chose what is foolish in the world to shame the wise; God chose what is weak in the world to shame the strong; God chose what is low and despised in the world, even things that are not, to bring to nothing things that are, so that no human being might boast in the presence of God.
Romans 9:23-24 (ESV)
in order to make known the riches of his glory for vessels of mercy, which he has prepared beforehand for glory—even us whom he has called, not from the Jews only but also from the Gentiles?
Romans 8:33 (ESV)
Who shall bring any charge against God’s elect? It is God who justifies.
John 6:37-39 (ESV)
All that the Father gives me will come to me, and whoever comes to me I will never cast out. For I have come down from heaven, not to do my own will but the will of him who sent me. And this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up on the last day.
John 6:44-45 (ESV)
No one can come to me unless the Father who sent me draws him. And I will raise him up on the last day. It is written in the Prophets, ‘And they will all be taught by God.’ Everyone who has heard and learned from the Father comes to me—
John 15:16 (ESV)
You did not choose me, but I chose you and appointed you that you should go and bear fruit and that your fruit should abide, so that whatever you ask the Father in my name, he may give it to you.
John 15:19 (ESV)
If you were of the world, the world would love you as its own; but because you are not of the world, but I chose you out of the world, therefore the world hates you.
John 17:2 (ESV)
since you have given him authority over all flesh, to give eternal life to all whom you have given him.
Matthew 24:22 "And except those days should be shortened, there should no flesh be saved; but for the ELECT'S sake those days shall be shortened."
Matthew 24:24 ". . . insomuch that, if it were possible, they shall deceive the very ELECT."
Matthew 24:31 ". . . and they shall gather together HIS ELECT from the four winds . . . "
Romans 9:11 "For the children being not yet born, neither having done any good or evil, that the purpose of God according to ELECTION might stand, not of works, but of him that calleth. "
Romans 11:5, 7 "Even so then at this present time also there is a remnant according TO THE ELECTION OF GRACE. What then? Israel hath not obtained that which he seeketh for; but THE ELECTION hath obtained it, and the rest were blinded. "
Romans 11:28 ". . . but as touching the ELECTION, they are beloved for the fathers' sakes. "
2 Tim. 2:10 "Therefore I endure all things for the ELECT'S SAKES, that they may also obtain the salvation which is in Christ Jesus with eternal glory."
Titus 1 ". . . according to the faith of God's ELECT. . . "