I don't normally read Leighton Flowers because he's a hack. Ironically, I think it's unfair to assess freewill theism by spokesmen like Flowers. However, a Facebook notification directed me to a post of his. He begins by quoting Rom 5:2 as follows:
Christ through whom we have gained access by faith into this grace in which we now stand.
He then says Calvinists tacitly reword the verse as follows:
Christ through whom we have gained access by grace into this faith in which we now stand.
Several issues:
i) Flowers is too lazy to present an actual argument. It's not my job to make his argument for him. But for discussion purposes, let's try to tease out the implicit argument. This seems to be what he's thinking: according to Calvinism, even faith is the product of God's grace. On that view, grace precedes faith.
Flowers imagines that Rom 5:2 contradicts that understanding because it reverses the order: faith precedes grace. According to Calvinism, grace is the gateway to faith. According to Flowers, faith is the gateway to grace. Assuming that's the thrust of what Flowers has in mind:
i) His argument hinges on the syntactical relationship between faith and grace in Rom 5:2. There is, however, a text-critical issue. As a recent commentator has noted:
"By faith" is disputed among the major manuscripts, and probably does not belong in the original text–although this is probably a reasonable scribal hypothesis regarding Paul's belief. S. Porter, The Letter to the Romans (Sheffield 2015), 115.
So Leighton's entire argument hinges on the presence of a word which may well be a scribal interpolation. It's at least as likely if not more likely that Paul never used that word in 5:2.
ii) In addition, Flowers commits a few related semantic fallacies. He acts as though "grace" has the same sense every time Paul uses the word. In addition, he acts as though the mere use of that Greek word carries the entire concept of grace in Pauline theology. By the same token, he acts as though the truth of Reformed theology depends on words having the same meaning in systematics and dogmatics that they have in biblical usage. So his semantic fallacy is several layers deep.
iii) In context, Paul seems to be using "grace" as a synonym for a justified state. And that's consistent with Reformed theology. According to Calvinism, justification is contingent on faith.
In Reformed theology, "grace" has multiple phases. The fact that one stage of saving grace is subsequent to faith doesn't mean there can't be prior stages of grace. In Reformed theology, the concept of grace encompasses salvation from start to finish, viz. election, redemption, regeneration, saving faith, justification, adoption, sanctification, perseverance, glorification. The fact that some stages of grace are post-conversion doesn't militate against pre-conversion stages of grace.