1:17 How are we to understand the reference to fear here?
1:10-12 What do we learn here?
We learn from this that even though the Old Testament prophets foretold God's plan of redemption which was to come through the sufferings of Christ, they did not know to whom, or when, it would happen, nor of the glory to follow the suffering.
All they knew was that it was not for them, nor for their time (cp Gen 3:14-15; 49:1, 10; Isa 52:13 - 53:12).
The angels also desire to look into God's redemptive plan. They observe believers constantly and even now are being taught by the church the manifold wisdom of God (cp 1Cor 4:9; 11:4-10; Eph 3:9-11; 1Ti 5:21).
Principalities and powers in Eph 3:10 refers to both good and bad angels
(See also comments on 1Cor 11:3-16, Eph 3:9-12).