The secret of Victory is simply Christ himself in the heart of the believer. This truth, of Christ's indwelling, is, and always has been, a mystery.

Romans 16:25.
Ephesians 3:9 with Colossians 1:26, 27.
Ephesians 5:30, 32 (R. V.).
Colossians 4:3.

Christ himself taught this truth.

John 14:20, 23; 15:1-7; 17:21-23.
Matthew 28:20.
Revelation 3:20. (See also Mark 16:20).

It was a vital reality to the Apostle Paul.

Romans 8:10.
1 Corinthians 6:15.
1 Corinthians 12:27 (R. V.).
2 Corinthians 5:17.
2 Corinthians 13:5.
Galatians 2:20.
Galatians 3:27.
Galatians 4:19.
Ephesians 3:17.
Philippians 1:21
1 Thessalonians 5:10.
Hebrews 3:6.

The words "in Christ," which recur in many other passages, will have a new literalness when read in the light of the above.

The Apostle John had a like conception of Christ's indwelling presence.

1 John 2:28 to 3:6, 24.
1 John 4:4, 12, 13, 16.
1 John 5:20.