St. Thomas on Justification - Grace and Remission of Sins

It is not possible to obtain the remission of sins apart from the grace of God. But as we saw in a previous post, justification is the remission of sins. If it is not possible, then, to receive remission of sins apart from grace, then it is not possible to be justified apart from grace, and those who say that Catholics believe a "works-based" gospel are badly misinformed.

[B]y sinning a man offends God as stated above (Question 71, Article 5). Now an offense is remitted to anyone, only when the soul of the offender is at peace with the offended. Hence sin is remitted to us, when God is at peace with us, and this peace consists in the love whereby God loves us. Now God's love, considered on the part of the Divine act, is eternal and unchangeable; whereas, as regards the effect it imprints on us, it is sometimes interrupted, inasmuch as we sometimes fall short of it and once more require it. Now the effect of the Divine love in us, which is taken away by sin, is grace, whereby a man is made worthy of eternal life, from which sin shuts him out. Hence we could not conceive the remission of guilt, without the infusion of grace. [ST I-II, Q113, A2; emphasis added]

Also with respect to the baselessness of the charges that many Protestants make against the Gospel, it's not enough to just stop sinning.

As Augustine says (De Nup. et Concup. i, 26), if to leave off sinning was the same as to have no sin, it would be enough if Scripture warned us thus: "'My son, hast thou sinned? do so no more?' Now this is not enough, but it is added: 'But for thy former sins also pray that they may be forgiven thee.'" For the act of sin passes, but the guilt remains, as stated above (Question 87, Article 6). [ST, op. cit., ad 3]