About speaking the truth in love, A true love means you will tell people the truth, if you love them, you want them to know the truth.
If you do not love them, then you will not tell them the truth. And telling lies is showing a type of hatred of the truth towards another, and people speak lies unawares, even with sincerity, but a lie it still is, God will be the judge.
We can verify we know the truth when what we say is in agreement with the Word of God, and we have that written down for us, we know the entire counsel of God in the law, his prophets, the New testament scriptures is true and no lie for God cannot lie.
Which also shows God loves us as he has given to us His truth.
If someone comes to us and does not have this testimony, then he is a liar and an evil worker. Christ will tell them this,
22 Many will say to Me in that day, ‘Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name?’ 23 And then I will declare to them, ‘I never knew you; depart from Me, you who practice lawlessness!’
The Certainty of God’s Witness
6 This is He who came by water and blood—Jesus Christ; not only by water, but by water and blood. And it is the Spirit who bears witness, because the Spirit is truth. 7 For there are three that bear witness [
b]in heaven: the Father, the Word, and the Holy Spirit; and these three are one. 8 And there are three that bear witness on earth: the Spirit, the water, and the blood; and these three agree as one.
9 If we receive the witness of men, the witness of God is greater; for this is the witness of [
c]God which He has testified of His Son. 10 He who believes in the Son of God has the witness in himself; he who does not believe God has made Him a liar, because he has not believed the testimony that God has given of His Son. 11 And this is the testimony: that God has given us eternal life, and this life is in His Son. 12 He who has the Son has [
d]life; he who does not have the Son of God does not have life. 13 These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, [
e]and that you may
continue to believe in the name of the Son of God.
Beware of Antichrist Deceivers
7 For many deceivers have gone out into the world who do not confess Jesus Christ
as coming in the flesh. This is a deceiver and an antichrist. 8 Look to yourselves, that [
c]we do not lose those things we worked for, but
that we may receive a full reward.
9 Whoever [
d]transgresses and does not abide in the doctrine of Christ does not have God. He who abides in the doctrine of Christ has both the Father and the Son. 10 If anyone comes to you and does not bring this doctrine, do not receive him into your house nor greet him; 11 for he who greets him shares in his evil deeds.