Yes, shocking I know. This is always evolving a bit, but the other day, I realized I finally had an actual short answer to this. Saying it was all just a mystery was not satisfying.
So here goes:
Calvinism, as articulated by documents such as the Westminster Confession or by, for example, Charles Spurgeon, are withing the bounds of Christian orthodoxy.
Arminianism, as articulated by Jacob Arminius or by, for example, Charles Wesley, are within the bounds of Christian orthodoxy.
Calvinism, as articulated by, for example, A.W. Pink is in grave danger of falling outside the bounds of Christian orthodoxy.
Arminianism, as articulated by, for example, Charles Finney, is in grave danger of falling outside the bounds of Christian orthodoxy.
If I were to widen the statement a bit, I would add:
Soteriology and a theory of free-will and predestination, as articulated in the Catechism of the Roman Catholic Church are within the bounds of Christian orthodoxy.
The functional practice and proclamation of these doctrines throughout the Roman church’s history has been rather spotty, both geographically and in different eras. (Late pre-reformation Europe and south America come to mind)
Soteriology and a theory of free-will and predestination, as articulated by the Lutheran, Anglican and Orthodox churches is also within the bounds of Christian orthodoxy.
Soteriology and a theory of free-will as generally articulated by present-day liberals, including many (not all!) American Lutherans, Episcopalians, and Presbyterians, amounts to little more than universalism. This means that it falls outside the bounds of Christian orthodoxy.