Careful reading and belief of the Scriptures, over a significant period of time.
It took a while for all of it to sink in.
I saw "eternal security" early on, probably in the early '80's ( I've been a believer since 1978, when I heard the Gospel in a "Traditionalist" Independent Baptist church during a revival meeting at the age of 12 ).
Then I saw individual election in 2003 after a friend of mine showed me three passages:
Romans 8:29-30
Ephesians 1:4-5.
2 Thessalonians 2:13-14.
I had been a believer for about 25 years before I saw these, due to my neglect of the Scriptures over that time period.
Added to that I found:
Psalms 65:4.
John 6:29.
John 6:64-65.
John 10:26
Acts of the Apostles 13:48.
and many others.
My favorite verse is in my signature.
But if I had to choose another, it would be the one
@Particular has listed.
Like him, I've not read much of John Calvin, if at all.
I've only recently begun listening to men like Rolfe Barnard, DJ Ward, Voddie Baucham, Henry Mahan, Don Fortner and others on Sermon Audio / Free Grace Radio, and I find their preaching very refreshing in these last days.
But, I've never felt that I needed anything other than God's word, truthfully.
Preaching like these men do is simply icing on the cake.