Abiding in Christ causes confidence in our salvation, and the presence of God’s Spirit brings assurance! It is “By the Spirit” we know that God abides in us.

1 John 3:23-24

23 This is His commandment, that we abelieve in the name of His Son Jesus Christ, and love one another, just as He commanded us.

24 The one who keeps His commandments abides in Him, and He in him. We know by this that He abides in us, by the Spirit whom He has given us.

New American Standard Bible: 1995 Update (La Habra, CA: The Lockman Foundation, 1995), 1 Jn 3:23–24.