There is some ambiguity in the Greek manuscripts as to whether or not this phrase should be kept by
or kept for.
The New Testament speaks of those who are being kept for the day of the Lord whether they be Christians (John 17:11–12; 1 Thessalonians 5:23; 1 Peter 1:4–5) or unbelievers destined for judgment (2 Peter 2:9; 2 Peter 3:7; Jude 1:6, Jude 1:13). This phrase is making the point that Christian believers are being guarded by God so that one day when Christ returns they will be given to him.
1 Jude, a servant of Jesus Christ and brother of James, To those who are called, beloved in God the Father and kept for Jesus Christ: