Believers in works based "gospels" often object to the concept of Sola Fide (faith alone) because they misunderstand it. In the Bible, it is completely indisputable that salvation is by grace, through faith, not of works. Why? Because the Bible explicitly says "by grace you have been saved through faith, (...) not of works". So, obviously, we're saved by grace through faith, not of works. It is especially helpful to couple this with Romans 11:6, which says "And if by grace, then it is no longer of works; otherwise grace is no longer grace. But if it is of works, it is no longer grace; otherwise work is no longer work."
So, clearly, works just don't enter the equation. You can be saved by grace, you can believe (falsely) that you can be saved by works, but under no circumstances can you add the two together. When you add works to salvation, grace ceases to be grace.
Because this is such an easy thing to establish with scripture, the heretic's only weapon is philosophy. "But if we're saved by faith alone, that means we don't need works at all, so even a really bad Christian can be saved." Now, first off, theoretically, this is true. In fact, we're just barely talking theoretically. Here's what Paul says about that in Romans 4:5-8:
"But to him who does not work but believes on Him who justifies the ungodly, his faith is accounted for righteousness, just as David also describes the blessedness of the man to whom God imputes righteousness apart from works: “Blessed are those whose lawless deeds are forgiven, And whose sins are covered; Blessed is the man to whom the Lord shall not impute sin.”" (Cf. Psalm 32:1-2)
Now, keeping in mind David slept with a married woman, then had her husband killed to cover it up, I'd say there's quite the margin for error. So yes, theoretically, you can be a bad Christian and be saved. In fact, on the flip side, we're all bad Christians anyway, we all sin (1 John 1:8), and to offend even once is to be guilty with regard to all (James 2:10). But although it is theoretically correct, those who argue against Sola Fide in this manner are still arguing against a straw man version.
See, in order to understand what it means to be saved by grace through faith, we need to ask what grace and faith are. Grace, as we've already established, is the antithesis of works. Grace is unmerited favour. While we deserve God's wrath, He instead offers us life. Good is offered where bad is owed. This is received by faith, so we need to ask, what is faith?
Those who argue against Sola Fide do so on the fundamental misunderstanding that faith is mere intellectual assent. As James repeats in his epistle, faith without works is dead. Think of it this way: My first camera was powered by AA batteries. AA batteries carry a certain amount of charge, after which they "die", and cease to function. Put a dead battery in my camera, it will not work. The camera is powered by batteries alone, but the batteries need to be "alive" before my camera can take a single photo.
In much the same way, salvation is by faith, but it is only a certain kind of faith that saves. In this scenario, works are represented by the pictures. My camera is not powered by its pictures, it's powered by "living" batteries that enable it to take the pictures in the first place. In the same way, living faith is what enables us to do good works.
So you see that just because we don't need good works to be saved doesn't mean we don't do good works after we are saved. Salvation, in and of itself, does not require you to do anything you couldn't do with your hands and feet nailed down (Luke 23:42-43). But supposing you're not being executed by the Romans? In this case, you have opportunities to take your salvation to its next step: The works God prepared for you ahead of time. Faith does not just involve acknowledging that God exists, but actually having the will to let Him do as He pleases with you.
It is obvious, then, which is the superior Gospel model. Salvation by faith + works is explicitly denied throughout the scriptures. Works necessarily negate grace, because then salvation ceases to be a gift, but a wage. By contrast, salvation is universally said to be received by faith alone. However, that faith, while it could theoretically remain alone, is naturally partnered with works. Faith is only truly alone when something physically stops works from accompanying it, such as a deathbed conversion. If faith is literally all you can offer God, you will still be saved. And if, like David, you sin, yet you still have genuine faith and confess, God is faithful and just to forgive those sins. You have an advocate with the Father. But let's be honest here: If you have genuine faith, you are not going to squander it. Gradually, the one who has true faith will be conformed to the image of Christ.