God is a Spirit:
and they that worship Him
must worship him in Spirit and in truth.
–Jesus in John 4:24 (KJV)–

Remain in me, as I also remain in you. 
No branch can bear fruit by itself;
it must remain in the vine.
Neither can you bear fruit unless you remain in me.
–Jesus in John 15:4 (NIV)–

Dr. Thomas C. Oden, John Wesley’s Teachings, 4 vols (Zondervan, 2014).

Adam Clarke, Clarke’s Christian Theology (Schmul Publishing, 1990).

P. C. Nelson, Bible Doctrines (Gospel Publishing House, 2009).

Ernest S. Williams, Systematic Theology, 3 vols (Gospel Publishing House, 1953).

Mack Tomlinson, In Light of Eternity: The Life of Leonard Ravenhill (Free Grace, 2010).

Gary Wilkerson, David Wilkerson (Zondervan, 2014).