Hmm, don't know the source of the quote, but it is worthy of questions. Certainly, the one true God can be known in a general, natural sense through His Creation. OTOH, God can only be known in a personal manner if He so chooses to reveal Himself to us (in whatever manner He sees fit). That is precisely the claim of Christianity, that He revealed Himself to us through the Incarnation of His eternal Son, and the founding of the Church, etc., etc.
Though I doubt that man needs any agents to lead him into evil, which God creates, as One.
Somewhat baffling. Creation is one, but Evil is not a created "thing". Evil represents a "lack", or disorder, in God's Creation. Time is a part of Creation and will come to an end as Creation runs it's course to the fulfillment of the goals that God has set for it.
From a Christian perspective, the effects of Original Sin are those which turn our incomplete nature at war with itself. That is, our physical side against our spiritual faculties (mind and will). St. Paul refers to our lower nature as "the flesh" whereas this includes our emotions, etc. pitted against our minds and wills.
This war allows for easy temptation among various sources including one another, an inordinate desire for temporal goods over eternal ones, and the evil spirits who try to sway us through these.
From a Christian perspective, it is the Grace of God obtained through the Life, Death, and Resurrection of our Lord Christ Jesus, and given through (primarily) the sacraments, in union with our free consent of mind and will by Faith, that we work out our salvation, crushing evil down in the process. From God's perspective, glimpsed at through His revelation to us, this process is worked out "till the close of the age" with each of us playing a role, either for good or evil. And it is in this perspective that one can say that evil ultimately will work out for the overall good.
Evil is, in a sense, a necessary concept if for no other reason that our - and the angels - freedom is at stake. God has given us the ability to reject Him and the Good that He has planned for us. His Graced is never "forced" on us. We have to say "Yes".