“Grace be with all them that love our Lord Jesus Christ in sincerity.” (Ephesians 6:24, emphasis mine)
If you’re not sure, just ask. Nothing worse than going on in blindness, not knowing that you’re somehow missing the Lord altogether through some feigned authenticity. Paul closes his letter to the Christians in Ephesus with that caveat. Nearly all his canonical letters end on a grace note, so to speak. It’s Ephesians that tacks on those last two words.
“Therefore take heed to your spirit, that ye deal not treacherously.” (Malachi 2:16b)
While that part of Malachi is repeated from the verse prior to it (2:15), and both deal with a husband’s conduct towards his wife in remaining true to her in heart and mind, I think the same admonition could be leveled at those who profess Christ. There is a deep work that necessarily takes place in the heart and mind of those who “love our Lord Jesus Christ in sincerity” to where you know. You know that you know the one who “loved [you] and gave himself for [you]” (Galatians 2:20) and are beyond convinced that you are in fellowship with him. Because it’s something that is living and active, if I may.
“Though the Lord be high, yet hath he respect unto the lowly: but the proud he knoweth afar off.” (Psalm 138:6)
Again, if you’re not sure as to your level of sincerity (or lack thereof), just ask. When you first believed is when you received “the adoption of sons [and daughters].” (Galatians 4:5b) In other words, the same status that Christ possesses by virtue of being “the firstborn among many brethren.” (Romans 8:29b) did you receive. I have to say, that a prolonged period of disconnect through the willful ignoring of him is a position that no child of his should even consider approaching. But again, reestablishing what you had when you first met him is as simple as a childlike acknowledgement or conversation. Find something to thank him for that he has done for you in the past week or so. This is how sincerity is built, one thought at a time.
“If thou sayest, Behold, we knew it no; doth not he that pondereth the heart consider it? And he that keepeth thy soul, doth not he know it? And shall not he render to every man according to his works?” (Proverbs 24:12)
Authenticity has to be the hardest thing to fake. How would one even go about doing that? As an image presented necessarily needs not just a model but also a sounding board (audience), authenticity on the order of Christ must have him in view. When Jesus said “Blessed are the pure in heart: for they shall see God.” (Matthew 5:9) he implies that when one is transparent in heart, they’ll see through everything to the Father. Point is, there is a level of sincerity and authenticity that Jesus builds in to you when you endeavor to live as did he: with reference to God. And if you’re having trouble with your sincerity, he’ll help you. Meanwhile, act like him. Be kind and thoughtful and present with those whom you encounter. Lift people up before the Lord and pray for them and you’ll look back at intervals and see yourself becoming the person God thought of when he first made—and then remade (see 2 Corinthians 5:17)—you.
“And hereby we know that we are of the truth, and shall assure our hearts before him. For if our heart condemn us, God is greater than our heart and knoweth all things.” (1 John 3:19-20)