What was the main belief behind calvinism?

Answer 1

The total sovereignty of God. Calvinism as a movement is defined by the mnemonic:

TULIP

T= Total Depravity (that we are so constrained by our fallen nature that we will not choose to persue God without His gracious intervention - See Romans 1-3)

U= Unconditional Election (considering the above, we can never warrant salvation and nor would we even desire it, so God chooses to call his chosen elect to faith, not because they are more deserving but simply because He chose them for His good pleasure - See Romans 8, Ephesians 1-2)

L= Limited Atonement (Christ died for his elect in a specific sense - while all may benefit by His death, only those who come to faith (that is to say, the elect) have received atonement for their sins - See John 6, 10)

I= Irresistable Calling (while we may resist God's calling, he is able and willing to overcome our resistance if we are part of his chosen people - see John 6-10)

P= Perseverance of the Saints (that those who are genuinely chosen will persevere in faith to the end, bearing fruit in accordance with their calling - sometimes summarised as "once saved, always saved" - that a genuine call to faith will result in God preserving us to the end - See John 10, Romans 8)

This is considered controversial in how it relates to free will and what it means for those who never come to faith. Limited atonement is a controversial point even in Calvins theology, in that he would affirm that Christ did die for the world at numerous points. Nevertheless, it is a fair summary of his teaching on salvation, considering it's brevity.