Twice John says that "you have overcome the evil one" in V13 and 14. The second is the key to understanding this idea. Note the full sentence of 1 John 2:14b -
I have written to you, young men, because you are strong, and the word
of God abides in you, and you have overcome the evil one.
Observe that these young men have two characteristics that qualify them for haing overcome the evil one:
1. They are strong
This is direct allusion to two other statements of Paul which allude to an OT passage was well:
- Rom 1:11, 12 - For I long to see you so that I may impart to you some spiritual gift to strengthen you, that is, that you and I may be mutually encouraged by each other’s faith.
- Eph 6:10 - Finally, be strong in the Lord and in his mighty power.
Paul then goes on to show how one becomes "strong in the Lord" by using the full armor of God such as, belt of truth, breastplate of righteousness, shoes of the Gospel, shield of faith, helmet of salvation, sword of the Spirit, namely, the Word of God.
Being strong in the Lord is a direct allusion to Zech 10:12 which says:
I will strengthen them in the LORD, and in His name they will walk
2. The Word of God abides in you
This is actually part of the full armor of God as listed above which Paul calls, "The Sword of the Spirit".
It is only when we are "strong in the Lord", covered with the "full armor of God" that as weak humans we can say that we have overcome the evil one. This makes the Lord's prayer all the more important where Jesus taught us to pray, "deliver us from the evil one".
Now, we do not pray to be delivered from the evil one and then refuse to wear the full armor of God - it is only in the "strength of the Lord" that such a statement is possible. Jesus summarized this when he said, "For apart from Me you can do nothing." (John 15:5)