Here's a few concise reasons why I'm not a Calvinist.
Number one is the doctrine of unconditional election, which essentially teaches that there
is no condition for salvation.
God predestines certain for heaven, others for hell, based upon his whim.
Though quote Ephesians 1 verse 11 where it states, In whom also we have obtained an inheritance,
being predestinated according to the purpose of him, who worketh all things after the counsel
of his own will.
However, verse 12 states that we should breathe to the praise of his glory, who first trusted
in Christ, proving that it's not according to his whim, but rather an individual's
willingness to believe on Jesus Christ, which is an open invitation for all.
Romans 10 13 states, For whosoever shall call upon the name of the Lord shall be saved.
Number two is the doctrine of limited atonement, which essentially teaches that the blood of
Jesus Christ is only efficacious for some, but not for all.
However, 1 John 2 2 states that he is the propitiation for our sins, and not for ours
only, but also for the sins of the whole world.
1 Timothy 4 10 states that he is the savior of all men, but especially of those that believe.
Full sermon on Reformed Theology in the description.