Something interesting that I have noticed in scripture is that we are often referenced as being lost sheep, lost coins, gone astray, etc... 

What I have realized with this wording is that for something to have gone astray or be lost, it means that it is no longer where it was intended. If the elect were always intended for glory and the reprobates were always intended for destruction. Then are we all lost? Would the non-elect be precisely where they are intended?

How could a reprobate be lost if they are right where God wants them? If we are classified as lost and astray, then what have we been lost from if not the intent of being with God?

If Calvinism were true, would not our entry into the kingdom simply be perceived by God as, "I have added you in"; not, "I have found what was lost"? The only way this wording works is if God had always intended for humanity, all of humanity, to be in relation with him. 

I believe this is clearly taught in the story of the prodigal son which is often misconstrued to reference a backsliding Christian. Jesus in this parable, and the two others just before, was when he was challenging the Pharisees of why he was eating with sinners. In context, this seems to clearly be a story of initial repentance, not the redemption of a backsliding Christian.

A Luthern brother has challenged me in this interpretation of the Prodigal Son with the following question: 

Find one verse in the bible that refers to non believers as sons or to have received an inheritance from the Father 

His question is to challenge that we should not be viewed as restored into God's kingdom, but as rather new inhabitants who were never intended to be there.

Here is my response:

Paul calls us all God’s offspring in Acts 17:22-34, and he is talking to strictly unbelievers in Areopagus

Acts 17:29-31

29 Being then God's offspring, we ought not to think that the divine being is like gold or silver or stone, an image formed by the art and imagination of man. 30 The times of ignorance God overlooked, but now he commands all people everywhere to repent, 31 because he has fixed a day on which he will judge the world in righteousness by a man whom he has appointed; and of this he has given assurance to all by raising him from the dead.” [Emphasis Added]

We are all created in his image with the intent of populating the nations where all of humanity was supposed to be in right relationship with God. Because of original sin, we are dead (separated from relationship). Still, offspring, but no longer in relationship to be called his children and we are now children of the devil and slaves to sin(John 3:10). We all are lost, and like sheep have gone astray. Notice we are considered lost and astray because the perfect place for everyone was to be with God. The parable of the prodigal son was Jesus defending his interaction with sinners. Although the lessons learned from the prodigal can apply to one who is backsliding, there is no grounded biblical argument that it doesn’t apply to an initial sinner first.

Similarly, in Romans 9:1-5 Paul even specifically relents to the Jews who are still in rejection saying, 

Romans 9:4-5 

They are Israelites, and to them belong the adoption, the glory, the covenants, the giving of the law, the worship, and the promises. 5 To them belong the patriarchs, and from their race, according to the flesh, is the Christ, who is God over all, blessed forever. Amen. (ESV)

This sounds like quite a list of gifts for the Jews to have "squandered" like the prodigal son. Paul even specifically uses the word adoption here. Other translations even use the word sonship.

God has given all of humanity the gift of life, breath, dominion over his creation, and being made in his image. These can easily represent the gifts given to the prodigal which was squandered through sin and man is now dead/separated/no longer a child.

This aligns with why the Bible calls us lost and astray, because we are not where we belong, separated from God. To the Jews first, and then to the Gentiles. God is drawing ALL(John 12:32), called ALL(Matthew 22:14), and commands ALL(Acts 17:30) to repent and to turn to him to be in the right restoration of where they ought to be. 

No one in sin is where they belong and God genuinely desires for all to turn to him and for their proper relationship to be restored. (2 Peter 3:9)