A Christian is someone who has received the Holy Spirit because “Christ” or “Messiah” means “anointed one” and the anointing is the Holy Spirit. Romans 8v9 states:-“ . . . you are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his.”So, it's vital to receive the Holy Spirit, but he is invisible and we cannot trust our own hearts that we are right with God . . ."The heart is deceitful above all things, and desperately wicked: who can know it?" (Jer. 17v9)(Jesus warns that many who call Him "Lord" will be told "I never knew you", he's talking about church-goers, not agnostics !). So, God must bare independant witness, the apsotles waited for this, . . .In the new testament it is known precisely when people receive the Holy Spirit, and when they hadn't. The way of salvation hasn't changed.At Pentecost, the Holy Spirit began to be given as never before, making people sons of God (see John 7v39, 14v17, 20, 16v7). The apostles reported:-“ . . . they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance. . . having received of the Father the promise of the Holy Ghost, he has shed forth this, which you now SEE and HEAR. . . For the promise is unto you, and to your children, and to ALL that are afar off, even AS MANY AS the Lord our God shall call.” (Acts 2v4, 33, 39)If you were told that what you could "see and hear" (the speaking in tongues) was the promised Holy Spirit, and that this was to you and all who God would call, wouldn’t you expect to speak in tongues, or be told if this was not the case?With the gentiles:-“ While Peter yet spake these words, the Holy Ghost fell on all them which heard the word.” (Acts 10v44)How did he know ?“For they heard them speak with tongues, and magnify God.” (v46)If it is possible to receive the Spirit and NOT speak in tongues, the apostles were *wrong* to judge that the Spirit had just fallen upon people when they spoke in tongues, logically they could only say that people had *at some unknown time previously* received the Spirit . . . but they didn't.