John 15:4 - 11
ESV - 4 Abide in me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in me. 5 I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Once we trust Jesus alone as the only way of salvation, He seals us with the Holy Spirit of promise. Ephesians 1:13-14. In whom ye also trusted, after ye heard the word of truth, the gospel of your salvation, (First Corinthians 15:1-4) in whom also AFTER ye believed, ye were sealed by that Holy Spirit of promise.
Second Timothy 2:13 If we believe not, He abideth faithful, He can not deny Him self. His Holy Spirit that lives in all believers.
Hebrews 13:5 He will never leave us or forsake us. He will never cast us our. John 6:37