Adam Clarke on 1 John 2:5

But whoso keepeth his word - Conscientiously observes his doctrine, the spirit and letter of the religion of Christ.

Is the love of God perfected - The design of God's love in sending Jesus Christ into the world to die for the sin of man τετελειωται, is accomplished, in that man who receives the doctrine, and applies for the salvation provided for him. This seems to be the meaning of the apostle.

That we are in him - That we have entered into his spirit and views, received his salvation, have been enabled to walk in the light, and have communion with him by the Holy Spirit.