John 15:9-11
9 As the Father loves me, so I also love you. Remain in my love.
10 If you keep my commandments, you will remain in my love, just as I have kept my Father's commandments and remain in his love.
11 I have told you this so that my joy may be in you and your joy may be complete.
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If I want Christ's joy in me, I am to keep his commandments. By doing that, I remain in his love, and my joy will be complete.
Nothing more to say. It's that simple. The gate may be narrow and the path difficult, but it is very clear.
