The biggest problem with Reformed theology is that it contradicts scripture and doesn't understand some of the basic mechanics of soteriology.
For example, Paul spoke about salvation in Romans 10 when he said:
[Rom 10:1-2, 5-17 NASB] 1 Brethren, my heart's desire and my prayer to God for them is for [their] salvation. 2 For I testify about them that they have a zeal for God, but not in accordance with knowledge. ... (in other words, they are zealous, but they don't know what they are talking about -- much like Reformed Theologists)
5 For Moses writes that the man who practices the righteousness which is based on law shall live by that righteousness. 6 But the righteousness based on faith speaks as follows: "DO NOT SAY IN YOUR HEART, 'WHO WILL ASCEND INTO HEAVEN?' (that is, to bring Christ down), 7 or 'WHO WILL DESCEND INTO THE ABYSS?' (that is, to bring Christ up from the dead)." 8 But what does it say? "THE WORD IS NEAR YOU, IN YOUR MOUTH AND IN YOUR HEART"--that is, the word of faith which we are preaching, 9 that if you confess with your mouth Jesus [as] Lord, and believe in your heart that God raised Him from the dead, you will be saved; 10 for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation. 11 For the Scripture says, "WHOEVER BELIEVES IN HIM WILL NOT BE DISAPPOINTED." 12 For there is no distinction between Jew and Greek; for the same [Lord] is Lord of all, abounding in riches for all who call on Him; 13 for "WHOEVER WILL CALL ON THE NAME OF THE LORD WILL BE SAVED." 14 How then will they call on Him in whom they have not believed? How will they believe in Him whom they have not heard? And how will they hear without a preacher? 15 How will they preach unless they are sent? Just as it is written, "HOW BEAUTIFUL ARE THE FEET OF THOSE WHO BRING GOOD NEWS OF GOOD THINGS!" 16 However, they did not all heed the good news; for Isaiah says, "LORD, WHO HAS BELIEVED OUR REPORT?" 17 So faith [comes] from hearing, and hearing by the word of Christ.
Paul says he wants to tell them about salvation... then he proceeds to quote heavily from Deuteronomy 30. So lets see what Deuteronomy 30 has to say regarding "the righteousness based on faith."
[Deu 30:1, 6, 11-15, 19 NASB] 1 "So it shall be when all of these things have come upon you, the blessing and the curse which I have set before you, and you call [them] to mind in all nations where the LORD your God has banished you, ... 6 "Moreover the LORD your God will circumcise your heart and the heart of your descendants, to love the LORD your God with all your heart and with all your soul, so that you may live. ... 11 "For this commandment which I command you today is not too difficult for you, nor is it out of reach. 12 "It is not in heaven, that you should say, 'Who will go up to heaven for us to get it for us and make us hear it, that we may observe it?' 13 "Nor is it beyond the sea, that you should say, 'Who will cross the sea for us to get it for us and make us hear it, that we may observe it?' 14 "But the word is very near you, in your mouth and in your heart, that you may observe it. 15 "See, I have set before you today life and prosperity, and death and adversity; ... 19 "I call heaven and earth to witness against you today, that I have set before you life and death, the blessing and the curse. So choose life in order that you may live, you and your descendants,
3 times in this chapter on "the righteousness based on faith" God says He has set the choice for life & death, blessing & cursing before them. He says it is not too difficult for them to choose (this contradicts the primary tenet of Reformed Theology). He says "Don't even say in your heart" that someone can't choose unless indwelled by heaven. This choice is not made in heaven. He says the choice is near to us ... in our mouth and in our heart, and whomever chooses life will be saved. This passage even contains what Jesus said (Matt 22:37, Mark 12:30, Luke 10:27) was the greatest commandment in all the law in Deuteronomy 30:6 - to love the lord with all you heart, soul, and strength. We also know from Deu 30:6 this chapter is about the salvation and righteousness based on faith because it talks about circumcision of the heart (which is the indwelling of the Holy Spirit according to Rom 2:28-29).
Furthermore, if you look at Deu 30:19 we can see the synergy between God and man in that it says "I call Heaven (God) and earth (man) to bare witness (to testify) of the choice man makes for salvation. This is in total agreement with another verse that describes this synergy:
[Rom 8:15-17 NASB] 15 For you have not received a spirit of slavery leading to fear again, but you have received a spirit of adoption as sons by which we cry out, "Abba! Father!" 16 The Spirit Himself testifies with our spirit that we are children of God, 17 and if children, heirs also, heirs of God and fellow heirs with Christ, if indeed we suffer with [Him] so that we may also be glorified with [Him.]
The Spirit himself (heaven) testifies (bares witness) with our spirit (earth) that we are children of God. Adopted as heirs. This synergy is critical.