Nathan,
The article which Susan referred to by Martyn Lloyd-Jones is excellent and you would benefit from taking time to read it. To answer your question directly as to what you are missing is that things aren't as simple as you would perhaps like them to be, or at least according to how you posed the issue. Salvation is by grace through faith in Christ alone. However, each of those terms has meaning that includes various things. For example, when we say we are saved by grace;
Sola Gratia, we are confessing that we believe that God in His sovereign mercy, from all eternity predestined a remnant of fallen mankind to salvation in Christ. In that decree, the
means to that end are included, e.g., regeneration, justification by faith, imputation of Christ's active obedience, sanctification and glorification. All is of God. When we speak of "faith", this too is pregnant with meaning. The modern church has so dumbed down the biblical essence of faith that it hardly has any resemblance to that faith which is flows out of the new heart of flesh created at regeneration. To "call upon Christ" is far more than saying a few "magic words", e.g., "Oh Jesus! Come into my heart." We are to believe
IN Christ; the person of Christ. The false gospel says to "believe that Jesus died for you". This is a far cry from what true faith does; i.e., reaches out and takes hold of the
PERSON of JESUS. For one of the finest explanations of what it means to be "Justified by faith", you simply MUST read Dr. Joel Beeke's article found here:
Justification by Faith Alone: The Relation of Faith to Justification.
Lastly, you asked why can't an Arminian or a Charismaniac be saved? The simple answer is that if a person who embraces Arminianism
in his/her heart, i.e., if that person truly believes what Arminian teaches then it is impossible for that person to be saved because Arminianism denies
Sola Gratia, Sola Fide, Solus Christus! That system of doctrine teaches that salvation is "synergistic", i.e., God and man working together, which is a flat contradiction of GRACE ALONE. However, because of the nature of how the sin nature effects our minds and hearts, it is possible for one to
profess something but not really believe it. As you are aware, it works both ways. One can profess wrong doctrine but actually embrace true doctrine in the heart. And one can profess true doctrine but have no spiritual life.
It is for this reason that I wrote that little article to help people discern what it is the TRULY believe, at least intellectually and then compare that to what they are trusting in. See that article here:
Do You REALLY Believe that Salvation is by Grace Alone?.
In His Grace,