There was a time when Saul of Tarsus thought he knew God and that he was serving Him with zeal and earnestness. The kind of God he imagined he knew permitted him to kill his opponents and think that he was doing God a service. Then one day, he met with the true God who opened his eyes to see Him, and Saul became a completely different man! He realised that he hadn't truly known God till then, and gave the rest of his life to getting to know Him in every aspect of doctrinal and experiential knowledge (Php.3:8,10). He understood now that many of his fellow Jews were also mistaken as he was, even though they were full of zeal for God in their own way (Ro.10:1,2).
Is it possible that some of us may be going on in life with religious zeal and yet we don't know God in the right way? We may be zealously loyal to our church and very active in serving God in the way our church teaches. But can we ignore the possibility that our church may also be wrong in different ways and our leaders misled in their understanding or practice. We cannot tell ourselves that God will notice our sincerity and all will be fine. Don't we need to individually make sure that we are following God with true knowledge apart from sincerity?
Of course, knowing God is a never-ending growth. But we need to make sure that we are going in the right direction. I would suggest two tests we can apply to ourselves to check our direction.
One test is to ensure that we are always willing to learn and receive correction. For this, we need to recognise the fact that we don't know everything (1Co.13:12) which also implies that some things we think we know may be wrong or imperfect.
The second test is to see if we are increasingly becoming able to 'love' others, including those who disagree with us. It's not about liking them or what they do, but realising that the difference between us and them is far smaller than the difference between us and God! After all, they are all our fellow creatures even with many differences with us.
If we have any 'holier than they' attitude towards those from other churches or even other faiths it only shows we don't have a proper understanding of our own standing before God as recipients of grace. Saul was 'blameless according to the Law' and yet his attitude towards Christians at that time showed how far he was from God. If we have a 'we and they' attitude towards others there's something seriously lacking in our life even if we are full of knowledge and have a great name. God has not been able to teach us true humility. Jesus was able to be with prostitutes and tax collectors along with the right attitude because He loved them. Of course, He did not become like them, but He gave us an example of humility along with His purity.
The Pharisees kept their distance from 'sinners'. Jesus was angry with them because they thought they were great scholars and saints even when they lacked compassion on needy and sinful people (Mt.9:13).