We cannot ascribe our repenting and believing to our own wisdom, humility, sound judgment or good sense, but, rather, to Christ alone. We turn our trust from ourselves to Christ only because He first opened our eyes, unplugged our ears and turned our stoney heart to a heart of flesh (Ezek 36:26) that we might believe the gospel. (Deut 29:4, 30:6) Arminian prevenenient grace actually begs the question - if two persons receive the same prevenient grace, why does one man believe the gospel and not the other? What makes them to differ? It is obviously not grace which makes them to differ since both had grace so all that is left is some native good will or good inclination that the other did not have. But who makes the will good? Where does the wisdom or humility to come to Christ come from? No man is naturally willing to submit to the humbling terms of the gospel. The Bible declares it: Jesus says, "the Spirit quickens, the flesh counts for nothing...that is why I told you that no one can come to me unless the Father who sent me grants it." (John 6:63, 54). and just before this Jesus also declares "All that the Father gives me will come to me" (John 6:37)
So together in the texts Jesus in no uncertain terms declares 2 things: 1) that NO ONE can come to faith in Him UNLESS granted by the Father, and that through the quickening work of the Spirit AND 2) ALL whom He so grants will come to faith in Him. It leaves no room for the synergistic view. But you say "God commands all people to repent and believe the gospel." This is true... and its in the Bible ... the gospel summons to all people without exception but apart from grace NO ONE responds to it positively... left to themselves all people turn aside from the one true God. (See Rom 2, 3; and 1 Cor 2:14) And "No one can say "Jesus is Lord" apart from the Holy Spirit" (1 Cor 12:3).
So is it (even partly) by our own doing that we are in Christ? No it is..."...by His doing you are in Christ Jesus, who became to us wisdom from God, and righteousness and sanctification, and redemption, so that, just as it is written, “Let him who boasts, boast in the Lord.” 1 Cor 1:30-31
We repent, yes, and we believe, but it is God who, by his great mercy, changed our hardened heart to do so. It was not our natural wisdom or humility or good will that set us apart but it was the grace of God in Jesus Christ which granted all of these. Salvation is of the Lord.