Hebrews 6:9-12 (ESV)
Though we speak in this way, yet in your case, beloved, we feel sure of better things—things that belong to salvation.  For God is not so unjust as to overlook your work and the love that you showed for his sake in serving the saints, as you still do.  And we desire each one of you to show the same earnestness to have the full assurance of hope until the end,  so that you may not be sluggish, but imitators of those who through faith and patience inherit the promises.
The writer expresses great confidence in those he is writing by saying that he is sure of better things for them, things that accompany salvation. He assures them that God would never be unjust and forget to reward their loving works of service to the saints. But this does not mean that he has no concerns for certain of them.
A church can have a deserved reputation of being solid and faithful and loving and hardworking. But that will not necessarily mean that everyone in that church deserves it. There are those who will attach themselves to a vibrant church and take thanks directed at the church as being for them personally. The writer knows that even though as a group the Hebrew believers are doing well in many respects, there are those among them who are coasting on the reputation of the church as a whole.
How many people attend a good church and feel the blessing of God upon it and misinterpret that to mean that they themselves are doing fine when in fact they are doing nothing of the sort? The author knows that not all are fit for the praise that he has been lavishing on the group.
There is no hope so distressing than the false hope that some possess because of the faithfulness of their church. They look around themselves and see so much good being done but they never look inward and see that they are absent of the very things that they feel a part of.
While we need to put the rugged individualism of our culture far from the culture of the church, it remains a truth that every believer must stand alone before the judgement seat of Christ and give an account of himself to God. The good preaching, love for others, giving of money and evangelistic fervour that the church exhibited will do him absolutely no good. He needs to be a part of the good that the church is doing or he will have to answer to God for his lack of faithfulness.
There is no faithfulness by proxy and there is no reward for those who act as if there is. God gives gifts to all His children. Each one is blessed in order to be a blessing.
Do not be satisfied with being a member of a good church. You need to be a part of the reason the church is good. God knows your heart. You cannot hide behind the efforts of others. There are no tailgaters in heaven. You need to imitate those who have demonstrated faith before you and labour for the Lord.