Strongest Texts For Free Grace Theology

 List goes from top to bottom, the list starts with weaker ones and ends with the strongest texts:

2 Corinthians 11:3

But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ.

This verse implies Free grace theology, by two points. Firstly, it says that we can be "corrupted" from the simplicity that is in Christ, which refers to apostatizing from the gospel. It is called "simplicity" which implies faith alone, the gospel is simple, "believe on the Lord Jesus Christ and thou shalt be saved". But we may still fall from the gospel later in life.

There is a variant in this text, the Greek word used "ἁπλότητος", by itself refers to "simplicity", however in the Alexandrian texts, it adds the words "ἁγνότητος", which makes the previous word more likely to mean "sincerity". 

The Byzantine majority text (including the Peshitta) and the Latin manuscripts support the reading "simplicity", while the Alexandrian texts say "sincerity and purity".

1 Samuel 28:19

Moreover the LORD will also deliver Israel with you into the hand of the Philistines. And tomorrow you and your sons will be with me. The LORD will also deliver the army of Israel into the hand of the Philistines.”

In the context, Saul the king had fallen away to a sinful state, he persecuted David and done much other sins. Yet, he and his sons will be with Samuel. Some object that "with me" may just refer to generally the realm of the dead, yet his sons, who were way more godly were implied to go to the same place with Saul, thus heaven.

1 Corinthians 3:3

For ye are yet carnal: for whereas there is among you envying, and strife, and divisions, are ye not carnal, and walk as men?

In 1 Corinthians 3:3 Paul affirms the category of "carnal Christian", as he in other places affirms the Corinthians as saved.

Ephesians 1:13-14

In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory.

According to this verse, we have the Holy Spirit in such a way that we cannot lose Him, we are "sealed", which in Greek is "σφραγίζω", which in Greek means:

/sphragízō ("to seal") signifies ownership and the full security carried by the backing (full authority) of the owner. "Sealing" in the ancient world served as a "legal signature" which guaranteed the promise (contents) of what was sealed.

HELPS Word-studies

Thus Paul is saying that we are guaranteed salvation, without conditions, we will be saved in the end.

Romans 8:31-39

This verse doesn't even need commentary due to how clear it is:

31 What then shall we say to these things? If God is for us, who can be against us? 32 He who did not spare His own Son, but delivered Him up for us all, how shall He not with Him also freely give us all things? 33 Who shall bring a charge against God’s elect? It is God who justifies. 34 Who is he who condemns? It is Christ who died, and furthermore is also risen, who is even at the right hand of God, who also makes intercession for us. 35 Who shall separate us from the love of Christ? Shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword? 36 As it is written: “For Your sake we are killed all day long; We are accounted as sheep for the slaughter.” 37 Yet in all these things we are more than conquerors through Him who loved us. 38 For I am persuaded that neither death nor life, nor angels nor principalities nor powers, nor things present nor things to come, 39 nor height nor depth, nor any other created thing, shall be able to separate us from the love of God which is in Christ Jesus our Lord.