Can Salvation be Lost? Eternal Security vs. Conditional Security

In American Christianity especially, there is much confusion on the doctrine of eternal security (sometimes referred to as “once saved, always saved“).

Some circles of Christianity, for example, might teach that one can lose his/her  salvation and fall into the error of trying to keep his salvation by personal effort. According to scripture, this is a serious error (Gal. 3:1-3).

On the other hand, some circles teach that one can never lose his/her salvation, and therefore fall into the error of sinning willfully because he/she believes salvation can't be lost no matter how much an individual sins. This is also a serious doctrinal error (Heb. 10:26).

It would seem that some Scriptures suggests that God preserves those who are truly His (John 10:27-28; Heb. 13:5; Matt. 7:21-23; 1 John 2:19; Rom. 8:38-39), yet there is also an indication in scripture that in the journey of pursuing Christ, many will fall away from the faith (2 Pet. 2:1; Gal. 5:4; Heb. 10:26; 6:4-6; Ps. 69:28).  

So which is it? Are we eternally secure? Or is our salvation conditional on our own ability to walk in obedience? How do we understand apostasy, or those who once professed faith in Christ but now don’t?