I think many have answered how.
John's epistles are filled with hows.
1 John 4:15 "Whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God."
1 John 3:23-24 "And this is his commandment, That we should believe on the name of his Son Jesus Christ, and love one another, as he gave us commandment. And he that keepeth his commandments dwelleth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he hath given us."
John 14:17 "Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you."
John 6:40 "And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day."
There are honestly too many to even post. It is everywhere.
I would be terrified right now if I did not have any assurance of salvation.