Jude says that he "gave all diligence to write...". Jude was a diligent man. He is found praying after Christ's ascension with the apostles, disciples, and his brethren (Acts 1:14). He gave all his diligence because he knew the task was so important. God had inspired him with a message to write to the people he cared for, and the message was urgent. There was no time to waste. Jude's points, seen in this verse and the next, are that Christians must "earnestly contend for the faith" because of the "certain men crept in unawares". In Deuteronomy 6:7, the Hebrew people are told to diligently teach God's laws to their children, so that obedience to God's Word would continue, just as in Jude's day. Joshua tells the 2 1/2 tribes to take diligent heed to do the laws. Also in Deuteronomy 19:8, judges are told to inquire diligently to ensure a correct trial outcome and maintain order in society. We are told to keep our hearts with all diligence (Prov. 4:23), that diligently seeking God will bring favor (Prov. 11:27), and to diligently consider what is before us when dining with a king (Prov. 23:1). Paul was diligently saying and found (II Tim. 1:17), and wished for Timothy to do his diligence to come see him (II Tim. 4:9).
The application: Psalm 64:6 shows that the wicked can also be diligent in their wickedness. We must be sure to be diligent, but like Apollos (Acts 18:25), do it in serving and teaching the Lord. Do it first because He diligently sought us (Luke 15:8), second because our faith must be contended for (v. 3), and third because He is a "rewarder of them that diligently seek Him" (Heb. 11:6).