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.

whoso.

1 John 2:3,4 And hereby we do know that we know him, if we keep his commandments...

Psalm 105:45 That they might observe his statutes, and keep his laws. Praise you the LORD.

Psalm 106:3 Blessed are they that keep judgment, and he that does righteousness at all times.

Psalm 119:2,4,146 Blessed are they that keep his testimonies, and that seek him with the whole heart...

Proverbs 8:32 Now therefore listen to me, O you children: for blessed are they that keep my ways.

Proverbs 28:7 Whoever keeps the law is a wise son: but he that is a companion of riotous men shames his father.

Ecclesiastes 8:5 Whoever keeps the commandment shall feel no evil thing: and a wise man's heart discerns both time and judgment.

Ezekiel 36:27 And I will put my spirit within you, and cause you to walk in my statutes, and you shall keep my judgments, and do them.

Luke 11:28 But he said, Yes rather, blessed are they that hear the word of God, and keep it.

John 14:21,23 He that has my commandments, and keeps them, he it is that loves me: and he that loves me shall be loved of my Father...

Revelation 12:17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God...

Revelation 14:12 Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.

in him.

1 John 4:12,18 No man has seen God at any time. If we love one another, God dwells in us, and his love is perfected in us...

James 2:22 See you how faith worked with his works, and by works was faith made perfect?

hereby.

1 John 2:27,28 But the anointing which you have received of him stays in you, and you need not that any man teach you...

1 John 3:24 And he that keeps his commandments dwells in him, and he in him. And hereby we know that he stays in us...

1 John 4:13,15,16 Hereby know we that we dwell in him, and he in us, because he has given us of his Spirit...

1 John 5:20 And we know that the Son of God is come, and has given us an understanding, that we may know him that is true...

John 6:56 He that eats my flesh, and drinks my blood, dwells in me, and I in him.

John 15:5 I am the vine, you are the branches: He that stays in me, and I in him, the same brings forth much fruit...

Romans 8:1 There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.

1 Corinthians 1:30 But of him are you in Christ Jesus, who of God is made to us wisdom, and righteousness, and sanctification, and redemption:

2 Corinthians 5:17,21 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new...

Colossians 2:9,10 For in him dwells all the fullness of the Godhead bodily...

Thirtieth Day. The Unction from the Holy One.
And ye have an anointing from the Holy One, and ye know all things. And as for you, the anointing which ye received of Him abideth in you, and ye need not that any one teach you; but as His anointing teacheth you concerning all things, and is true, and is no lie, and even as it taught you, ye abide in Him.'--1 John ii. 20, 27. In the revelation by Moses of God's Holiness and His way of making holy, the priests, and specially the high priests, were the chief expression of God's Holiness in man. …
Andrew Murray—Holy in Christ

That to Him who Loveth God is Sweet Above all Things and in all Things
Behold, God is mine, and all things are mine! What will I more, and what more happy thing can I desire? O delightsome and sweet world! that is, to him that loveth the Word, not the world, neither the things that are in the world.(1) My God, my all! To him that understandeth, that word sufficeth, and to repeat it often is pleasing to him that loveth it. When Thou art present all things are pleasant; when Thou art absent, all things are wearisome. Thou makest the heart to be at rest, givest it …
Thomas A Kempis—Imitation of Christ

But Since no Man is Worthy to Come Forward in his Own Name...
But since no man is worthy to come forward in his own name, and appear in the presence of God, our heavenly Father, to relieve us at once from fear and shame, with which all must feel oppressed, [8] has given us his Son, Jesus Christ our Lord, to be our Advocate and Mediator, that under his guidance we may approach securely, confiding that with him for our Intercessor nothing which we ask in his name will be denied to us, as there is nothing which the Father can deny to him (1 Tim. 2:5; 1 John 2:1; …
John Calvin—Of Prayer--A Perpetual Exercise of Faith