Contradiction with Galatians 1:6
This verse addresses turning away from grace, while Acts 11:23 encourages continuing in it.
Galatians 1:6: I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
Contradiction with Hebrews 3:12
Warns against departing from God, whereas Acts 11:23 encourages cleaving unto the Lord.
Hebrews 3:12: Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.
Contradiction with James 1:8
Describes a double-minded man as unstable, contrasting with Acts 11:23's encouragement to purpose of heart.
James 1:8: A double minded man [is] unstable in all his ways.
Contradiction with 1 John 2:19
Talks about those who leave being not truly with them, unlike the message of steadfastness in Acts 11:23.
1 John 2:19: They went out from us, but they were not of us; for if they had been of us, they would [no doubt] have continued with us: but [they went out], that they might be made manifest that they were not all of us.