I'd like to make a few more quick points about Lee Irons in regard to John Piper:
1. One source of confusion is due to the fact that there are at least two conditions in play:
i) Faith is a necessary condition of justification
ii) Justification is a necessary condition of salvation
Lee oscillates between these two different, but interrelated, propositions.
2. I think some Calvinists are nervous about the word "condition" because they think it connotes uncertainly. Because "condition" is often used in human transactions, conditionality in that setting may be uncertain because human agents, unlike God, are not omnipotent and omniscient. Therefore, the parties cannot ensure the satisfaction of the conditions.
That, however, is not inherent in the nature of a condition. Rather, that's context-dependent. Incidental to the agent in question.
For instance, the eternal decree has conditionality. The Resurrection is contingent on the Crucifixion, which is contingent on the Incarnation. Likewise, the death of Christ is contingent on the action of human participants like Judas, Pilate, and the Sanhedrin.
That, however, doesn't render the outcome uncertain. These are determinate means to determinate ends. There's teleology within the decree, where the occurrence of one event depends on the occurrence of another event. Nested events; nested decrees. Yet the outcome is inevitable, as are the intervening events which facilitate the outcome.