Objection 3. Further, God's grace, by its advent, has no less effect in a believer than in an unbeliever. Now by coming to an unbeliever it causes the habit of faith. Therefore when it comes to a believer, who hitherto had the habit of lifeless faith, it causes another habit of faith in him. [ST II-II, Q4, A4]
Reply to Objection 3. Grace causes faith not only when faith begins anew to be in a man, but also as long as faith lasts. For it has been said above (I, 104, 1; I-II, 109, 9) that God is always working man's justification, even as the sun is always lighting up the air. Hence grace is not less effective when it comes to a believer than when it comes to an unbeliever: since it causes faith in both, in the former by confirming and perfecting it, in the latter by creating it anew. [ibid., ad 3; emphasis added]
God causes our faith, and God causes our justification; it is never caused by anything that we do, but rather by his grace. Those who say that we believe otherwise are badly misinformed. We cannot save ourselves.