Consider a prime employment opportunity. A kind-hearted boss offers you a job; an excellent job! There is always training for the employee. Remuneration is sufficient and the retirement plan is out of this world. The contract precludes dismissal. No matter how badly a job is done you can't get fired, though one is expected to eventually master the various tasks.
The point is obvious. Even if the boss has said he will never fire you—the contract does not allow for you to be fired—clearly the employee can quit.
One can take both the security passages and the apostasy passages at face value. God will never leave us, he helps us in our weaknesses, and he gives us strength to persevere. None of this prevents us from rejecting him. We can deny the faith and it says nothing of God's faithfulness
If we are faithless, he remains faithful—for he cannot deny himself. (2 Timothy 2)