To be sound in mind, we need to be strengthened by the Spirit in our inner man with the power of His might, His dunamis resurrection power.
How does that happen?
By being filled with the Spirit on a daily basis – drinking into the Spirit (1 Cor.12:13)(Eph.5:18).
As we worship the Lord in spirit and in truth, meditating on His Word and praying much in tongues, our inner man gets “overtaken” by the Spirit, being rooted and grounded in His love.
It is this agape love that drives out all fear, filling us with power to comprehend the breadth and length and depth and height of His love, coming to and experiential knowledge of his love, filling us with all His fullness. A sound mind is the inevitable result.