But does this mean that they were saved, or merely attending religious services and making an outward appearance, but their hearts were not changed?
The Pharisees thought they were believers, and followed the customs and rituals of the Jews, but when they came in contact with Jesus Christ, they were exposed for unbelievers that they were.
Indeed you can. We're not saved until we're in the boat. Until then we're still subject to drowning in sin.
Our life jacket is the Almighty's Grace, it helps keep your head above water while swimming toward that far of Shore.
You, my brothers and sisters, were called to be free. But do not use your freedom to indulge the flesh[a]; rather, serve one another humbly in love. For the entire law is fulfilled in keeping this one command: “Love your neighbor as yourself.” If you bite and devour each other, watch out or you will be destroyed by each other.
So I say, walk by the Spirit, and you will not gratify the desires of the flesh. For the flesh desires what is contrary to the Spirit, and the Spirit what is contrary to the flesh. They are in conflict with each other, so that you are not to do whatever you want. But if you are led by the Spirit, you are not under the law.
The acts of the flesh are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God.
But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law. Those who belong to Christ Jesus have crucified the flesh with its passions and desires. Since we live by the Spirit, let us keep in step with the Spirit. Let us not become conceited, provoking and envying each other.
Remember, Salvation is a double layered cake. While most of the icing is on the top layer, there is another layer called "Judgement Day" for those not "in Christ". Many forget this added complexity when discussing issues like the one above.
Much of the NT talks about not only being saved, but saved from Judgement Day as well.
Well:
"...who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come..."
Seems pretty clear. Takes a lot of gymnastics to turn that into a description of someone who has never believed.