Romans 5:6 — For when we were yet without strength, in due time Christ died for the ungodly.
without strength = still sinners (see verse 8).
In due time — according to God's plan, not circumstances.
For = on behalf of (not "instead of")
Ungodly = all, indicated by the lack of "the" in Greek — Christ died for all, being ungodly.
Notice the progression in these verses:
vs. 6 — without strength — our helplessness
vs. 8 — yet sinners — our sinfulness
vs. 10 — enemies — our willfulness