Here mentioned in the perfect tense, Jude also is writing unto those who are "preserved in Jesus Christ". This means that Christians are continually "kept" by God, all the way until the with of Jesus Christ, which Jude looks forward to in v. 21.
The application: In John 17, Jesus prays to His Father that He has kept His children for as long as He has lived on Earth (John 17:10-12). The only one that was lost was Judas, "the son of perdition", who was, like in Jude's purpose of writing, one who was numbered among the believers but never truly was. John would later explain this in his first epistle when speaking about antichrists- "They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us." (I John 2:19). Hence Christ's preservation is guaranteed to only His genuine children. But we know that this preservation spoken of is not about the temporal world, because many have been martyred for true faith. Rather Christ guarantees, and Jude writes to, genuine, blood-bought believers who shall "never perish, neither shall any man pluck them out of my hand" (John 10:22-30, where Jesus also tells the difference between false believers and real ones). The Holy Spirit in our lives is evidence of God's preserving power (Eph. 4:30).
Preserve (verb used with object)
to keep alive or in existence; make lasting: to preserve our liberties as free citizens.
to keep safe from harm or injury; protect or spare.
to keep up; maintain: to preserve historical monuments.
to keep possession of; retain: to preserve one's composure.
to prepare (food or any perishable substance) so as to resist decomposition or fermentation
Seal (verb used with object)
to affix a seal to in authorization, testimony, etc.
to assure, confirm, or bind with or as if with a seal: They sealed the bargain with a handshake. ("The Spirit itself beareth witness with our spirit, that we are the children of God..." -Rom. 8:16; II Cor. 1:21-22)
to impress a seal upon as evidence of legal or standard exactness, measure, quality, etc. (The sanctifying work of Christ after salvation)
to close by any form of fastening that must be broken before access can be gained. (Satan had to ask permission of God before doing anything to Job.)
to fasten or close tightly by or as if by a seal: She was sealing envelopes. My lips are sealed.